I'm assuming you use Ubuntu, which uses apt as the package management system. In that case "package not found" can mean two things (could be more, but these are common):
1) you misspelled the package name (when typing the command on command line)
2) the package you're trying to install/upgrade has been updated on the server, but your local package lists have not been updated, and the version you're trying to download no longer exists on the server
In the first case, simply make sure you do not misspell anything. In the second case, update your package lists (which you should do in any case) before attempting an install or upgrade action. To update the lists, click "refresh" or equivalent in the program you're using (like Synaptic or the "Add/Remove" tool), or from command line,
sudo apt-get update
After this, retry what you originally tried to do.
Third option would be that you're trying to get a package that does not exist. For example you may have copy-pasted a command off some webpage, but it's either old or for another distribution/version, and the package(s) mentioned in it do not exist on the repositories your system uses for fetching packages.