We've heard reports of this happening once in awhile, and it seems to be an issue with Steam itself. Some or all of the following steps have worked for other players:
Quit and restart Steam.
Restart your computer.
Right click the game in Steam, click Properties, click Local Files, then click Verify Integrity of Game Cache.
Log out of and back into Steam.
If you are playing offline, use the “Go Offline” feature of Steam.
Let us know if that does not help. You may also try contacting Steam support.