One of the steam games I have, Defense Grid (awesome game btw) requires me to verify integrity of game cache once in a while or else it crashes on startup. Yours may similarly have gotten corrupted and this is all you have to do through steam to get it to work again.
Go to your steam library, right click on the game's name, select properties, local files tab then click verify integrity of game cache. If you don't see the progress bar move within about 10 seconds, cancel and click verify again and it should work.