Having gone through several upgrades and countless repository’s added to the software sources, things can begin to get really messy really quickly and often begin to screw up when updating the system if you’re not careful. You may find yourself, like I have, wanting to restore your sources.list file back to its default in order to get things working again.
In order to do this there is a pretty handy webapp available called the “Ubuntu Sources List Generator” – http://repogen.simplylinux.ch/
By moving through each section it enables you to select your country, release and then check each repository you want to add, not only does it support all the official ones for things like security and updates, it also allows for 3rd party repos such as Banshee, Chromium, Conky and VLC amongst a bunch others. Once you have what you want hit generate and it’ll create a nice, comment, new sources.list file ready for you to replace your old one with.
You may want to backup your old sources.list just in case with the following command:
sudo cp /etc/apt/sources.list /etc/apt/sources.list.bkup
Once you have that you can open the sources.list and simply paste the new one in from the repogen:
sudo gedit /etc/apt/sources.list
Paste, Save and Close the file and now you should paste the generated GPG keys, found just below the generated sources.list file, into your terminal.
Everything should be back to default and allow you to once again update everything.