I had this problem today. I did this :
1. Load Steam
2. From the Library section, right-click on the game and select Properties from the menu.
3. Select the Local files tab and click the Verify integrity of game cache... button.
4. Steam will verify the game's files - this process may take several minutes.
Once the process is completed, the Check Window will automatically exit.
...and now it works. It's updated too so that might have had something to do with it!?
Hope this helps. It's annoying not being able to play when you've payed good money for it! Good luck!
1. Load Steam
2. From the Library section, right-click on the game and select Properties from the menu.
3. Select the Local files tab and click the Verify integrity of game cache... button.
4. Steam will verify the game's files - this process may take several minutes.
Once the process is completed, the Check Window will automatically exit.
...and now it works. It's updated too so that might have had something to do with it!?
Hope this helps. It's annoying not being able to play when you've payed good money for it! Good luck!

