I had this same problem, and for me it came from the game not getting the latest update automatically.
The fix that worked for me is: Go to the game in your library and right click on it, then go to properties. This will open a new window. Go to the "Local Files" tab at the top of this window and then click on "Verify Integrity of Game Cache". After that it should apply the newest update and start up just fine.
Thanks go to Ravagus for pointing this out to me.