If you are having problems with a Steam game, we generally recommend the following:
Click on "Library" and then "Games" on the Steam main screen.
Right-click on the game that is causing problems.
Select "Properties".
Click on "Verify integrity of game files..." under the "Local files" tab.
(If the integrity check fails to improve the situation,
delete the game folder in the Steam apps folder.)
Please note that this process can take a little while. As soon as the process is finished, you can restart your game as normal.