Changing some Software Update settings I possibly corrupted something, and now I don't see many packages in the list. For example:

alex@u120464:~$ sudo apt-get install codeblocks

[sudo] password for alex: 

Reading package lists... Done

Building dependency tree       

Reading state information... Done

E: Unable to locate package codeblocks

I checked all options in the Software Sources dialog, but packages are still not available. How can I fix this?

OS: Ubuntu 12.04, 64 bit.

Additional information.

alex@u120464:~$ sudo apt-get update

[sudo] password for alex: 

Ign http://extras.ubuntu.com precise InRelease

Ign http://security.ubuntu.com precise-security InRelease           

Ign http://archive.canonical.com precise InRelease                  

Ign http://archive.ubuntu.com precise InRelease

Ign http://archive.ubuntu.com precise-updates InRelease

...

It looks like most Ubuntu repositories are not searched, how can I restore default update behaviour?

Edit.

Finally, the problem was solved by copying /etc/apt/source.list file from another computer. This is the offending file content:

# deb cdrom:[Ubuntu 12.04.1 LTS _Precise Pangolin_ - Release amd64 (20120823.1)]/ dists/precise/main/binary-i386/



# deb cdrom:[Ubuntu 12.04.1 LTS _Precise Pangolin_ - Release amd64 (20120823.1)]/ dists/precise/restricted/binary-i386/

# deb cdrom:[Ubuntu 12.04.1 LTS _Precise Pangolin_ - Release amd64 (20120823.1)]/ precise main restricted



# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to

# newer versions of the distribution.

deb http://il.archive.ubuntu.com/ubuntu/ precise main restricted

deb-src http://il.archive.ubuntu.com/ubuntu/ precise main restricted



## Major bug fix updates produced after the final release of the

## distribution.



## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu

## team. Also, please note that software in universe WILL NOT receive any

## review or updates from the Ubuntu security team.



## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 

## team, and may not be under a free licence. Please satisfy yourself as to 

## your rights to use the software. Also, please note that software in 

## multiverse WILL NOT receive any review or updates from the Ubuntu

## security team.



## N.B. software from this repository may not have been tested as

## extensively as that contained in the main release, although it includes

## newer versions of some applications which may provide useful features.

## Also, please note that software in backports WILL NOT receive any review

## or updates from the Ubuntu security team.





## Uncomment the following two lines to add software from Canonical's

## 'partner' repository.

## This software is not part of Ubuntu, but is offered by Canonical and the

## respective vendors as a service to Ubuntu users.

deb http://archive.canonical.com/ubuntu precise partner

deb-src http://archive.canonical.com/ubuntu precise partner



## This software is not part of Ubuntu, but is offered by third-party

## developers who want to ship their latest software.

deb http://extras.ubuntu.com/ubuntu precise main

deb http://mirror.sov.uk.goscomb.net/ubuntu/ precise main universe restricted multiverse

deb-src http://mirror.sov.uk.goscomb.net/ubuntu/ precise universe main multiverse restricted #Added by software-properties

deb http://security.ubuntu.com/ubuntu/ precise-security universe main multiverse restricted

deb http://mirror.sov.uk.goscomb.net/ubuntu/ precise-updates universe main multiverse restricted

deb http://mirror.sov.uk.goscomb.net/ubuntu/ precise-backports universe main multiverse restricted

deb-src http://extras.ubuntu.com/ubuntu precise main