Steam won't boot up game even though it says I have correct activation code

I've been playing the game every single day through steam since launch with no problems. Today I boot the game up and it asks for an activation code again? I enter it in and it says this code is already in use? I view game CD code by right clicking on the game in steam copy yo clip board and try again, same problem. Both the code on steam and on my game manual are same. Can someone please tell mewhat on earth is going on now with steam and more importanly how do I play this game now?

You could try uninstalling, then reinstalling using unilock

People are seeing their mistake in using steam now

Have you tried playing the game from;

Program files > steam > steamapps > common >(then whatever FM file is then try the exe file)

I dont use steam but that seemed to work for other games when I got this problem with "steaming ****"