Пакеты не доступны для установки

Изменяя некоторые настройки обновления программного обеспечения, я, возможно, что-то испортил, и теперь я не вижу много пакетов в списке. Например:

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

Я проверил все опции в диалоге Software Sources, но пакеты по-прежнему недоступны. Как я могу это исправить?

ОС: Ubuntu 12.04, 64 бит.

Дополнительная информация.

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
...

Похоже, что большинство репозиториев Ubuntu не ищутся, как я могу восстановить поведение обновления по умолчанию?

Редактировать.

Наконец, проблема была решена путем копирования файла /etc/apt/source.list с другого компьютера. Это оскорбительное содержимое файла:

# 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