Sometimes when you join a server, you get something that says: "Error verifying STEAM UserID Ticket". This tutorial will teach you how to fix this problem without re-installing your Steam game.
Here is what you have to do:
NOTE: You will need to remember your account password otherwise you will not be able to log back in.
1) Log out of Steam - right click the Steam icon, click settings, and click "Login as a different user"
2) Close out of the Login window and open your Steam folder - usually Program Files\Valve\Steam
3) Delete a file called "ClientRegistry.blob"
4) Open Steam and it will say it is updating, and when that's done, log back in and you will be able to join servers again.