Sources missing in /etc/apt/sources.list, how do I get them back?
You seem to have removed all your software sources. To re-add them, launch the Software Center, then go to Edit -> Software Sources in the menu, and then check all the boxes under the Ubuntu Software tab:
Then click close, and the system will ask for your password and then generate a new source file for you. You probably won't need the Source code box checked:
- Do I really need the "Source Code" in my repository list?