Question #1: When you say you can't connect, do you mean a) you can't join servers/online games, or b) you can't even see any online games available? And what particular error messages are you getting from the specific games?
Not being able to connect to a particular server can mean any number of things, including problems not related to your computer connection (i.e. server doesn't allow "foreign" players on it, server is password-protected for a "private" game, etc.). Not being able to see any available servers could mean a problem with the connection, a problem on your PC, or even that you mistyped your username/password to log into the servers.
Question #2: If you haven't done so, are you able to connect using someone else's PC (i.e. your girlfriend's computer)?
Again, being able to log in on another PC, especially if using the same Internet connection as your PC, means that the problem is on your PC itself: application is blocked, bad installation, etc. If you still can't connect even on a different PC, there could be a problem with your online account for the game: mistyped username/password, account locked/disabled/deleted, etc.