synaptic does not work

Do you get an error message of any kind?

Are you able to connect to the internet and browse the web?

What is the output of Applications --> Accessories --> Terminal, then type this command:-

sudo apt-get update

Can you paste the results here?

shivasakthi@shivasakthi-desktop:~$ sudo apt-get update
[sudo] password for shivasakthi:
Ign cdrom://Ubuntu 11.04 _Natty Narwhal_ - Release i386 (20110427.1) natty InRelease
Ign cdrom://Ubuntu 11.04 _Natty Narwhal_ - Release i386 (20110427.1) natty Release.gpg
Ign cdrom://Ubuntu 11.04 _Natty Narwhal_ - Release i386 (20110427.1) natty Release
Ign cdrom://Ubuntu 11.04 _Natty Narwhal_ - Release i386 (20110427.1) natty/main TranslationIndex
Ign cdrom://Ubuntu 11.04 _Natty Narwhal_ - Release i386 (20110427.1) natty/restricted TranslationIndex
Err cdrom://Ubuntu 11.04 _Natty Narwhal_ - Release i386 (20110427.1) natty/main i386 Packages
  Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
Err cdrom://Ubuntu 11.04 _Natty Narwhal_ - Release i386 (20110427.1) natty/restricted i386 Packages
  Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
Ign cdrom://Ubuntu 11.04 _Natty Narwhal_ - Release i386 (20110427.1) natty/main Translation-en_IN
Ign cdrom://Ubuntu 11.04 _Natty Narwhal_ - Release i386 (20110427.1) natty/main Translation-en
Ign cdrom://Ubuntu 11.04 _Natty Narwhal_ - Release i386 (20110427.1) natty/restricted Translation-en_IN
Ign cdrom://Ubuntu 11.04 _Natty Narwhal_ - Release i386 (20110427.1) natty/restricted Translation-en
Ign http://deb.opera.com stable InRelease
Ign http://extras.ubuntu.com natty InRelease
Ign http://archive.ubuntu.com natty InRelease
Ign http://archive.ubuntu.com natty-updates InRelease
Ign http://archive.canonical.com natty InRelease
Ign http://archive.ubuntu.com natty-proposed InRelease
Ign http://security.ubuntu.com natty-security InRelease
Hit http://deb.opera.com stable Release.gpg
Hit http://archive.ubuntu.com natty Release.gpg
Hit http://extras.ubuntu.com natty Release.gpg
Hit http://archive.canonical.com natty Release.gpg
Ign http://dl.google.com stable InRelease
Hit http://security.ubuntu.com natty-security Release.gpg
Hit http://archive.ubuntu.com natty-updates Release.gpg
Hit http://extras.ubuntu.com natty Release
Hit http://archive.canonical.com natty Release
Hit http://deb.opera.com stable Release
Hit http://security.ubuntu.com natty-security Release
Hit http://archive.ubuntu.com natty-proposed Release.gpg
Hit http://extras.ubuntu.com natty/main i386 Packages
Hit http://archive.ubuntu.com natty Release
Hit http://archive.canonical.com natty/partner i386 Packages
Hit http://security.ubuntu.com natty-security/restricted i386 Packages
Hit http://deb.opera.com stable/non-free i386 Packages
Ign http://extras.ubuntu.com natty/main TranslationIndex
Hit http://archive.ubuntu.com natty-updates Release
Ign http://archive.canonical.com natty/partner TranslationIndex
Ign http://dl.google.com stable InRelease
Hit http://security.ubuntu.com natty-security/main i386 Packages
Hit http://security.ubuntu.com natty-security/universe i386 Packages
Hit http://security.ubuntu.com natty-security/multiverse i386 Packages
Ign http://security.ubuntu.com natty-security/main TranslationIndex
Ign http://security.ubuntu.com natty-security/multiverse TranslationIndex
Ign http://deb.opera.com stable/non-free TranslationIndex
Hit http://archive.ubuntu.com natty-proposed Release
Hit http://archive.ubuntu.com natty/main Sources
Hit http://archive.ubuntu.com natty/restricted Sources
Hit http://archive.ubuntu.com natty/multiverse Sources
Hit http://archive.ubuntu.com natty/universe Sources
Hit http://archive.ubuntu.com natty/main i386 Packages
Ign http://security.ubuntu.com natty-security/restricted TranslationIndex
Ign http://security.ubuntu.com natty-security/universe TranslationIndex
Hit http://archive.ubuntu.com natty/restricted i386 Packages
Hit http://archive.ubuntu.com natty/multiverse i386 Packages
Hit http://archive.ubuntu.com natty/universe i386 Packages
Ign http://archive.ubuntu.com natty/main TranslationIndex
Ign http://archive.ubuntu.com natty/multiverse TranslationIndex
Ign http://archive.ubuntu.com natty/restricted TranslationIndex
Ign http://archive.ubuntu.com natty/universe TranslationIndex
Get:1 http://dl.google.com stable Release.gpg [198 B]
Hit http://archive.ubuntu.com natty-updates/main Sources
Get:2 http://dl.google.com stable Release.gpg [197 B]
Ign http://extras.ubuntu.com natty/main Translation-en_IN
Get:3 http://dl.google.com stable Release [1,351 B]
Ign http://archive.canonical.com natty/partner Translation-en_IN
Hit http://archive.ubuntu.com natty-updates/restricted Sources
Ign http://extras.ubuntu.com natty/main Translation-en
Ign http://archive.canonical.com natty/partner Translation-en
Hit http://archive.ubuntu.com natty-updates/multiverse Sources
Hit http://archive.ubuntu.com natty-updates/universe Sources
Hit http://archive.ubuntu.com natty-updates/main i386 Packages
Hit http://archive.ubuntu.com natty-updates/restricted i386 Packages
Hit http://archive.ubuntu.com natty-updates/multiverse i386 Packages
Hit http://archive.ubuntu.com natty-updates/universe i386 Packages
Ign http://archive.ubuntu.com natty-updates/main TranslationIndex
Ign http://archive.ubuntu.com natty-updates/multiverse TranslationIndex
Ign http://archive.ubuntu.com natty-updates/restricted TranslationIndex
Get:4 http://dl.google.com stable Release [1,347 B]
Ign http://archive.ubuntu.com natty-updates/universe TranslationIndex
Hit http://archive.ubuntu.com natty-proposed/restricted i386 Packages
Hit http://archive.ubuntu.com natty-proposed/main i386 Packages
Hit http://archive.ubuntu.com natty-proposed/universe i386 Packages
Hit http://archive.ubuntu.com natty-proposed/multiverse i386 Packages
Ign http://archive.ubuntu.com natty-proposed/main TranslationIndex
Ign http://archive.ubuntu.com natty-proposed/multiverse TranslationIndex
Ign http://archive.ubuntu.com natty-proposed/restricted TranslationIndex
Ign http://archive.ubuntu.com natty-proposed/universe TranslationIndex
Get:5 http://dl.google.com stable/main i386 Packages [1,229 B]
Ign http://deb.opera.com stable/non-free Translation-en_IN
Ign http://deb.opera.com stable/non-free Translation-en
Ign http://dl.google.com stable/main TranslationIndex
Ign http://security.ubuntu.com natty-security/main Translation-en_IN
Ign http://security.ubuntu.com natty-security/main Translation-en
Ign http://security.ubuntu.com natty-security/multiverse Translation-en_IN
Ign http://security.ubuntu.com natty-security/multiverse Translation-en
Ign http://security.ubuntu.com natty-security/restricted Translation-en_IN
Ign http://security.ubuntu.com natty-security/restricted Translation-en
Ign http://security.ubuntu.com natty-security/universe Translation-en_IN
Get:6 http://dl.google.com stable/main i386 Packages [764 B]
Ign http://dl.google.com stable/main TranslationIndex
Ign http://security.ubuntu.com natty-security/universe Translation-en
Ign http://archive.ubuntu.com natty/main Translation-en_IN
Ign http://archive.ubuntu.com natty/main Translation-en
Ign http://archive.ubuntu.com natty/multiverse Translation-en_IN
Ign http://archive.ubuntu.com natty/multiverse Translation-en
Ign http://archive.ubuntu.com natty/restricted Translation-en_IN
Ign http://archive.ubuntu.com natty/restricted Translation-en
Ign http://archive.ubuntu.com natty/universe Translation-en_IN
Ign http://archive.ubuntu.com natty/universe Translation-en
Ign http://archive.ubuntu.com natty-updates/main Translation-en_IN
Ign http://archive.ubuntu.com natty-updates/main Translation-en
Ign http://archive.ubuntu.com natty-updates/multiverse Translation-en_IN
Ign http://archive.ubuntu.com natty-updates/multiverse Translation-en
Ign http://archive.ubuntu.com natty-updates/restricted Translation-en_IN
Ign http://archive.ubuntu.com natty-updates/restricted Translation-en
Ign http://archive.ubuntu.com natty-updates/universe Translation-en_IN
Ign http://archive.ubuntu.com natty-updates/universe Translation-en
Ign http://archive.ubuntu.com natty-proposed/main Translation-en_IN
Ign http://archive.ubuntu.com natty-proposed/main Translation-en
Ign http://archive.ubuntu.com natty-proposed/multiverse Translation-en_IN
Ign http://archive.ubuntu.com natty-proposed/multiverse Translation-en
Ign http://archive.ubuntu.com natty-proposed/restricted Translation-en_IN
Ign http://archive.ubuntu.com natty-proposed/restricted Translation-en
Ign http://archive.ubuntu.com natty-proposed/universe Translation-en_IN
Ign http://archive.ubuntu.com natty-proposed/universe Translation-en
Ign http://dl.google.com stable/main Translation-en_IN
Ign http://dl.google.com stable/main Translation-en
Ign http://dl.google.com stable/main Translation-en_IN
Ign http://dl.google.com stable/main Translation-en
Fetched 5,086 B in 17s (283 B/s)
W: Failed to fetch cdrom://Ubuntu 11.04 _Natty Narwhal_ - Release i386 (20110427.1)/dists/natty/main/binary-i386/Packages Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs

W: Failed to fetch cdrom://Ubuntu 11.04 _Natty Narwhal_ - Release i386 (20110427.1)/dists/natty/restricted/binary-i386/Packages Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs

E: Some index files failed to download. They have been ignored, or old ones used instead.
E: Could not get lock /var/lib/dpkg/lock - open (11: Resource temporarily unavailable)
E: Unable to lock the administration directory (/var/lib/dpkg/), is another process using it?
shivasakthi@shivasakthi-desktop:~$

is this another question?