How do I restore the default repositories?

You can use this trick. Open a terminal ( Pressing Ctrl+Alt+T ) and do these

• Move the corrupted one to the safe place

  sudo mv /etc/apt/sources.list ~/
  

  and recreate it

  sudo touch /etc/apt/sources.list
  

• Open Software & Updates

  software-properties-gtk
  

  This will open software-properties-gtk with no repository selected.

Then, change the server to Main server or to any other server of your choice. You must enable some repositories from the new window in order to create a new sources.list file in /etc/apt/.

• After enabling some sources from Ubuntu software tab, you can enable updates. To do so, switch to Updates tab and select one or more updates channel. I recommend selecting the security and updates channels at least. (This image is later added from Ubuntu xenial, so there can be some differences)

  

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Updated with inline content

This is the sources.list file for 12.04 Precise Pangolin.

###### Ubuntu Main Repos
deb http://archive.ubuntu.com/ubuntu/ precise main restricted universe multiverse 
deb-src http://archive.ubuntu.com/ubuntu/ precise main restricted universe multiverse 

###### Ubuntu Update Repos
deb http://archive.ubuntu.com/ubuntu/ precise-security main restricted universe multiverse 
deb http://archive.ubuntu.com/ubuntu/ precise-updates main restricted universe multiverse 
deb http://archive.ubuntu.com/ubuntu/ precise-proposed main restricted universe multiverse 
deb http://archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse 
deb-src http://archive.ubuntu.com/ubuntu/ precise-security main restricted universe multiverse 
deb-src http://archive.ubuntu.com/ubuntu/ precise-updates main restricted universe multiverse 
deb-src http://archive.ubuntu.com/ubuntu/ precise-proposed main restricted universe multiverse 
deb-src http://archive.ubuntu.com/ubuntu/ precise-backports main restricted universe multiverse 

###### Ubuntu Partner Repo
deb http://archive.canonical.com/ubuntu precise partner
deb-src http://archive.canonical.com/ubuntu precise partner

###### Ubuntu Extras Repo
deb http://extras.ubuntu.com/ubuntu precise main
deb-src http://extras.ubuntu.com/ubuntu precise main

If you're using another release, you need to replace the precise word with your Ubuntu release name. You can see which name you should use with this command:

lsb_release -c -s

To replace the word, you can use this sed command (assuming you copied the sources content in /etc/apt/sources.list):

sudo sed -i "s/precise/$(lsb_release -c -s)/" /etc/apt/sources.list

Note 1: the word deb and deb-src refers to the repository format. deb is for binary packages and deb-src is for source packages.

Note 2: Using # at the start of the line makes that line a comment. apt will ignore it, so any repositories mentioned on that line will be disabled.

Note 3: There are repository lines which includes all four components: main, universe, restricted, multiverse. You can disable one or more of them by removing the word.

Note 4: You can find some explanation of the repositories in this my other answer

Go to https://repogen.simplylinux.ch.

Select your Country and Ubuntu Release.

For the default set of repositories, you need to enable these repositories.

• All of the Ubuntu Branches repositories.
• Security - Important Security Updates.
• Security Sources Repository
• Updates - Recommended Updates
• Updates Sources Repository

If you want to install software from Canonical Partner Repositories (closed source software), enable the Ubuntu Partner Repositories (both of them).

Enable any 3rd party repository you wish to enable.

Now, click the Generate List button at the bottom of that page and you'll see your generated sources.list.

Replace the old sources.list with the new one

Run the following commands in a Terminal.

sudo mv /etc/apt/sources.list /etc/apt/sources.list.old
gksudo gedit /etc/apt/sources.list

Copy/paste all the text from the newly generated sources.list to this file. Save it and close Gedit.

Now, update apt.

sudo apt-get update

This will update your repository index with the current sources.list and then you can install any software using Software Center, Synaptic or apt-get.

By default there will be a backup for your sources.list file on the same directory itself.

sudo cp /etc/apt/sources.list.save /etc/apt/sources.list

Answer from similar question here: https://askubuntu.com/a/465210/150936