I'm using steam and Activation Manager just pops, asks me for the serial key(which is already typed in) and verifies 100%. But the thing is the game won't start afterwards.

What's wrong? I played this game just fine 3 days ago.

If it was fine 3 days ago then quit frustrating yourself and roll back to a restore point at least 4 days ago and it should be fine.

May try verify the integrity of the game cache files, just a thought in case something is corrupted.