I use multiple computers, several at work and several at home, all running Ubuntu. I don’t always upgrade all of them to the latest Ubuntu release. As you must have realized, Ubuntu has a release cycle in which when a release reaches end of life (EOL), the update sources are moved to an old release server, and your updates stop working. If you do not wish to solve the problem by upgrading Ubuntu to a supported release, which you should, then you should configure the sources correctly to point to that sever for updates. Here is how:
sudo gedit /etc/apt/sources.list
Then press CTR+H and replace the current server, for example “us.archive.ubuntu.com” with “old-releases.ubuntu.com”, click replace all, save with CTL+S and exit with ALT+F4.
If you are accustomed to the terminal shortcuts, the following line works
sudo sed -i -e 's/us.archive.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list
Followed by the usual
sudo apt-get update && sudo apt-get dist-upgrade
FYI, the easiest way to upgrade Ubuntu, especially when you want to jump a release (its not possible to upgrade from 13.04 to 14.04 through system tools) is by burning the image of the latest release and installing it, without replacing or formating the home partition, which should be a seperate partition.
Keeping an old unsupported release means that you dont get security updates and thats dangerous.
One thing I dont get though, even after pointing the sources to the old-release-server, updating sources and doing a dist-upgrade, do-release-upgrade doesnt find a newer release.