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

Try logging in to the website with your email address instead of your nickname, then changing your password and logging out. Often, that will clear the "incorrect login details" issue.