Have you tried to uninstall/reinstall the game itself?
- First is to uninstall/reinstall the game itself.
- Next to try is to follow this troubleshooting steps from the link below.
- If it doesn't work, here's another thread related to that concern that may help.
- Here's the link: http://steamcommunity.com/app/20920/discussions/0/522729901453128993/