DRM101 I had the same issue yesterday but later found out that MW3 on Steam had a update which screwed up the files. As far as I know this hasn't been fixed yet but the only way I know how to fix this problem is to somehow downgrade the game version. I would check posts from other people that discussed about this problem and see if they have a way to downgrade the game version or found a fix to this issue. I hope this info helped you understand as to why the game stopped working and I wish you the best of luck.