If you've been unable to play Crysis 2 multiplayer on PC, we have a tentative fix for you. Load up the game's multiplayer mode, enter your key, then go into single player, and finally, go back into multiplayer and re-enter your key. After this, you should be good to go and won't have to enter your key again.
Crytek is working on a fix for an upcoming second patch, so if you've not yet picked up the game, it should be no issue for you in the future if and when you do.
For less serious and less common issues, see this thread.