Maybe this is common sense, but I'm going to post it anyway because I had no idea.
Trying to launch MW2 this morning, I kept getting a 'This game is temporarily unavailable, please try again later' error. I have a kid, am a full time student, and have a full time job. I can't guarantee a 'later'. So I searched online for answers, which of course were: restart Steam, or restart your computer. I did both, with no change afterwards. I tried to launch other games, which worked fine, so I knew problem had to be with the game files, but Steam wasn't being too proactive when it came to fixing this for me.
So toying around, I found that if you right click on the game under 'Games' tab, go to Properties, then to the 'Local Files' tab, you can choose to 'Verify the integrity of game cache'. After doing so, sure enough Steam found some corrupt files and replaced them for me, and everything works fine now.
Just thought this may come in handy in the future, if not for somebody now.