Ubuntu 11.04 upgradation

In my production server(Ubuntu 11.04), When I try to install any package by running 'sudo apt-get install packagename' It gives error saying "E: unable to locate packagename". Can anyone help me to get out from this error. So I can't run 'apr-get upgrade'

I installed fresh Ubuntu 11.04, I have also found the same error there too

Thank you in advance.

Jamshed