Support for 11.04 is discontinued and you may have a difficult time completely restoring the sources.list file to your prior contents. Are you able to upgrade to a newer edition, perhaps with long term support? 12.04 LTS might be your best option, and 14.04 LTS comes out next month.
Try updating via the FromUSBStick instructions. I use this for system recovery, its a good tool to carry around in your backpack anyway. If your hardware is out of date and your worried about demand on your system, you can always do away with the 'fancier' Unity features and Compiz and such.
This blog post may be helpful
If upgrading isnt an option, you may be able to restore your sources.list as follows:
sudo sed -i 's,http://.*ubuntu.com,http://old-releases.ubuntu.com,g' /etc/apt/sources.list
sudo apt-get -y update
If you intend on rebundling the image, you will need to run the
following commands to ensure that bundled images retain the settings:
dpkg-divert --local --rename --add /etc/cloud/templates/sources.list.tmpl
sed -i 's,http://.*ubuntu.com,http://OLDRELEASEDOMAIN.com,g' \
| sudo tee /etc/cloud/templates/sources.list.tmpl
where OLDRELEASEDOMAIN is 'old-releases DOT ubuntu DOT com' (given that the url isnt allowed here, you are further encouraged to upgrade your system).