[cpnichol]

Does it say this when you try to connect to a server?

"error validating steam user id ticket"

This is usually a temporary problem that can be solved by exiting steam then reconnecting or a problem with the server, if it's the server then no one will be able to connect until the server is fixed or updated or something.

The steam support site also says:

If restarting Steam does not resolve the issue, please exit Steam and delete the following files:
[*]Steam.dll[*]SteamUI.dll[*]ClientRegistry.Blob [/ul]
These will be found under your Steam installation directory - by default, this will be C:\Program Files\Valve\Steam\ (this may vary if you specified a different installation folder, or if you have moved your Steam installation).