Uninstall the game completely and re-install it by doing the following:
- Open your Steam software and go to the LIBRARY tab, where you will find a list of games that you own on Steam.
- Find your game in the list and right-click on it. Next, select Uninstall... from the menu, which will uninstall the game.
Having uninstalled the game, select it again in the Library list and click the INSTALL button to the right of the list - this will initiate the download and installation process of the game.