After searching these forums, I've seen that other people also have this problem.
I can login to these forums just fine and can even change my password, but for some reason, I can't log in to Crysis 2 in-game. It tells me that I have incorrect login details, even if I'm sure they're right.
In fact, I was just playing Multiplayer last night, and even leveled up a few times, as you can see on this link showing my stats ---> http://c2stats.com/pc/p_WallDough
I bought the Maximum Edition of the game last year in Steam's Autumn Sale for $12 (an awesome price) and I still have the serial code.
Another thing that I noticed, however, is that my single player profiles have been deleted, even If I haven't touched any game files, but It's no big deal, I'm glad to replay the whole game as it's a really great one.
Now, I just hope somehow this can be solved