Why is this happening? I hadn't played for a few days, and I felt the urge tonight. But when I tried to sign in it said my serial key was invalid. I bought it on Steam the day it came out and I've never had this problem before. Why?!

reenter it

make sure you didnt share your cd key with other people otherwise they can use your key without you knowing, so go down the list of people you shared your cd key with and find out if they have been using it without your knowledge and ask them to stop so you can play it, and in the future make sure you dont share it with other people than yourself so this doesnt happen again

Thank you tuskan14. Since my serial code was still there when I hit sign in, I didn't think I'd need to re-enter it. But that seems to have worked.

I just bought a top of the line computer for the first time last October, because I wanted to play games looking their best. I had no idea it'd be such a headache.

its jus an after-patch known bug

its jus an after-patch known bug

And I read about it, but what I read didn't tell me my serial would still be IN there if I had to re-enter it. I may have just missed that part.