Ubuntu :: Updating System / Saying Update Information Is Outdated?

Ubuntu tried to update, but some problems occurred. The triangle with an exclamation point in it showed up and I rolled my mouse over it.This is what it says:

"The update information is outdated. This may be caused by network problems or by a repository that is no longer available. Please update manually by clicking on this icon and then selection 'Check for updates' and check if some of the listed repositories fail."

I did what it says and this came up:"The repository may no longer be available or could not be contacted because of network problems. If available an older version of the failed index will be used. Otherwise the repository will be ignored. Check your network connection and ensure the repository address in the preferences is correct."

and in the white box below the message:"Failed to fetch url 404 Not Found
Some index files failed to download, they have been ignored, or old ones used instead."