Packages are not available for installation

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http://askubuntu.com – 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. (HowTos)