Acer 5000 Scanner not detected

Unplug scanner and run command
   lsusb
Plug scanner, wait 30 seconds and run again command
   lsusb
You should see a new line with your scanner. Please post this line (with ID numbers).

System->Administration->Package Management (Synaptic)
Search xsane and install it.
Then run Applications->Graphics-->Xsane and check if xsane detect it.
Do the same with Applications->Graphics-->Simple Scan

farooq@ubuntu:~$ lsusb
Bus 005 Device 002: ID 04a5:20f8 Acer Peripherals Inc. (now BenQ Corp.) Benq 5000

This is the new line i get.

Linux kernel see your scanner. So could have a driver issue.
Did you check Xsane and Simple Scan ?

Yes. Xsane and simple Scan both checked. In simple scan it detected the scanner but when command is given to scan it shows "Failed to scan".

Can you post result of command
   scanimage -L
Can you provide exact and complete error given by Xsane ? Xsane is an older software, so it's more easy to search on Web solutions.
If you want to google, I advise you using "04a5:20f8" as a keyword.

http://www.sane-project.org/sane-mfgs.html#Z-BENQ
5000 USB 0x04a5/0x20f8 Good USB ID 0x04a5,0x20f8: Color / grayscale scans working up to 1200 DPI SnapScan (1.4) sane-snapscan

Have a look at http://cltb.ojuba.org/en/articles/scanner.html.
I'm sorry, but have no time to read this thread.