I downloaded the game off steam two weeks ago and the multiplayer worked fine every time i logged in. Then I didnt play it for 4 or 5 days and now I get an "invalid serial number"

plz help!

anyone?

also when I re-enter my serial number it still doesnt work..

Try this:

1. Start game
2. Then go to Multiplayer > Internet > Yes > Change Player Profile > Recover Internet Player Profile
3. Type your account name, password and serial