Ubuntu 13.04/13.10 - W: Failed to fetch [source] 404 Not Found

So you're running Ubuntu 13.04 or Ubuntu 13.10, or something else, and one day, when using apt-get, this happened:

W: Failed to fetch http://us.archive.ubuntu.com/...  404  Not Found [IP: 91.189.91.13 80]

W: Failed to fetch http://archive.ubuntu.com/...  404  Not Found [IP: 91.189.92.201 80]

W: Failed to fetch http://archive.ubuntu.com/...  404  Not Found [IP: 91.189.92.201 80]

E: Some index files failed to download. They have been ignored, or old ones used instead.

Has lightning struck all the servers?  What's actually happening is that the distro has reached EOL - end of life.

What EOL means is that a lot of applications are not supported or updated anymore.  It also means that when you do an apt-get update, you might get a whole bunch of 404's, and if you try to use the Software Center, it might hang or error out with "Failed to download repository information".

Repositories contain packages which are tested and built specifically  for your version of Ubuntu.  Eventually support dies out in favour of newer releases and long term support (LTS) releases.

One option is to upgrade.  Another option is to edit your /etc/apt/sources.list file like so:

That'll keep you synced with the old versions of the packages for your release.  I used vim with the text replace commands:
:%s/archive.ubuntu/old-releases.ubuntu/gc
:%s/security.ubuntu/old-releases.ubuntu/gc
(plus 'y' for every replacement).  You might need to adjust that depending on which server you're using.
 
More about Ubuntu repositories and servers:
https://help.ubuntu.com/community/Repositories/Ubuntu