how to terminate software updates/ update manager to be able to run it again?

Most probably is just the lock file that needs to be reset.
Can you confirm with:

1) Menu->Accessories->Terminal
2) ps -A

that there are no processes running using apt?

You can paste here the result of the command if you want us to have a look at it.

Hello,

in a terminal window execute the following command:

sudo fuser -v /var/lib/dpkg/lock

This should tell you which process is guilty.

to Cesare Tirabassi, here is the result of ps -A:

jillian@jillian-desktop:~$ ps -A
  PID TTY TIME CMD
    1 ? 00:00:01 init
    2 ? 00:00:00 ksoftirqd/0
    3 ? 00:00:00 watchdog/0
    4 ? 00:00:00 events/0
    5 ? 00:00:00 khelper
    6 ? 00:00:00 kthread
    8 ? 00:00:00 kblockd/0
    9 ? 00:00:00 kacpid
  107 ? 00:00:00 pdflush
  108 ? 00:00:00 pdflush
  110 ? 00:00:00 aio/0
  109 ? 00:00:00 kswapd0
  697 ? 00:00:00 kseriod
 1821 ? 00:00:00 khubd
 1922 ? 00:00:00 kjournald
 2148 ? 00:00:00 udevd
 2960 ? 00:00:00 shpchpd_event
 3033 ? 00:00:00 kgameportd
 3858 ? 00:00:00 acpid
 3948 ? 00:00:00 syslogd
 3974 ? 00:00:00 dd
 3976 ? 00:00:00 klogd
 4295 ? 00:00:00 gdm
 4307 ? 00:00:00 gdm
 4312 tty7 00:00:20 Xorg
 4334 ? 00:00:00 hpiod
 4338 ? 00:00:00 python
 4385 ? 00:00:00 cupsd
 4410 ? 00:00:00 dbus-daemon
 4425 ? 00:00:01 hald
 4426 ? 00:00:00 hald-runner
 4431 ? 00:00:00 hald-addon-acpi
 4487 ? 00:00:00 hald-addon-keyb
 4498 ? 00:00:00 hald-addon-stor
 4499 ? 00:00:00 hald-addon-stor
 4501 ? 00:00:00 hald-addon-stor
 4502 ? 00:00:00 hald-addon-stor
 4580 ? 00:00:00 hcid
 4585 ? 00:00:00 sdpd
 4594 ? 00:00:00 krfcommd
 4608 ? 00:00:00 mdadm
 4642 ? 00:00:00 atd
 4655 ? 00:00:00 cron
 4729 tty1 00:00:00 getty
 4730 tty2 00:00:00 getty
 4731 tty3 00:00:00 getty
 4732 tty4 00:00:00 getty
 4733 tty5 00:00:00 getty
 4734 tty6 00:00:00 getty
 4757 ? 00:00:00 dhclient3
 4770 ? 00:00:00 x-session-manag
 4812 ? 00:00:00 ssh-agent
 4815 ? 00:00:00 dbus-launch
 4816 ? 00:00:00 dbus-daemon
 4818 ? 00:00:00 gconfd-2
 4821 ? 00:00:00 gnome-keyring-d
 4823 ? 00:00:00 bonobo-activati
 4825 ? 00:00:00 gnome-settings-
 4827 ? 00:00:00 esd
 4836 ? 00:00:01 metacity
 4842 ? 00:00:01 gnome-panel
 4844 ? 00:00:01 nautilus
 4847 ? 00:00:00 gnome-volume-ma
 4855 ? 00:00:00 update-notifier
 4859 ? 00:00:00 gnome-cups-icon
 4862 ? 00:00:00 gnome-vfs-daemo
 4870 ? 00:00:00 gnome-power-man
 4880 ? 00:00:00 mapping-daemon
 4883 ? 00:00:00 trashapplet
 4890 ? 00:00:00 mixer_applet2
 4892 ? 00:00:00 clock-applet
 4896 ? 00:00:00 notification-da
 4909 ? 00:00:00 gnome-screensav
 4918 ? 00:00:00 firefox
 4929 ? 00:00:00 run-mozilla.sh
 4934 ? 00:00:33 firefox-bin
 4981 ? 00:00:00 kacpid-work-0
 4982 ? 00:00:00 kacpid-work-1
 5074 ? 00:00:00 gnome-terminal
 5075 ? 00:00:00 gnome-pty-helpe
 5076 pts/0 00:00:00 bash
 5096 pts/0 00:00:00 ps
jillian@jillian-desktop:~$

so is update manager running? it should be.. i think.thanks

to Ivan Szekeres, here it is:

jillian@jillian-desktop:~$ sudo fuser -v /var/lib/dpkg/lock
Password:
jillian@jillian-desktop:~$

there's no guilty process. what do i do now? thanks.

i ran sudo apt-get clean in the terminal, and after a list of updates, here's what i get:

E: dpkg was interrupted, you must manually run 'dpkg --configure -a' to correct the problem.
jillian@jillian-desktop:~$

how do i do that? thanks.

Open a terminal window (Menu->Accessories->Terminal), there you have to type the following:

sudo dpkg --configure -a

I was having the same problem. I was installing VirtualBox and accidentally didn't accept agreement. I closed the installer window and the thing froze. I did a CTRL ALT BACKSPACE and the installer wasn't hanging anymore. I tried to install it again and was getting that error about only one installer at a time.

This command solved it:
sudo dpkg --configure -a

Thanks.

Had the same issue. Tries to do updates using update-manager. Machine hung completely (left it for 2 hours). Only got it back via power cycling. Tried single updates at a time - no joy. Tried updates from command line - told me a package was severely inconsistent (Drupal, as it happens but that is almost certainly irrelevant). Couldn't remove or re-install Drupal to fix inconsistency.
THEN
On one of the (many) power cycles, I hit a 30-boot fsck on the root file system. That found errors (fixable). Booted back up & now have no problems.

Probably my own fault as this happened immediately after I moved swap partition. But I'll take it as a generic lesson for myself.

Advice: Do fsck on root partition.

 got a problem, 2 days back i was trying to install medibuntu repositories, from the command line
and from software sources(from third party software), typed in this command
sudo wget http://www.medibuntu.org/sources.list.d/gutsy.list -O /etc/apt/sources.list.d/medibuntu.list,
there is some thing like this to download from third party software started with .deb,
I typed that APT line and clicked ADD SOURCE button, and then it came up with an error message

E: Type '--16:53:40--' is not known on line 1 in source list /etc/apt/sources.list.d/medibuntu.list
E: The list of sources could not be read.
Go to the repository dialogue to correct the problem.
E: _cache->open() failed, please report.
And then after i realised my source list got corrupted, resulting am unable to access synaptic manager, update manager.
I should have shown u this today, sorry

Regards
santosh