Steam is validating game files
To validate a game’s cache, right-click the game in your Steam library and select Properties.
In the properties window, click the Local Files tab and click the Verify Integrity of Game Cache button.
You’ll find this file in your Steam directory – C:\Program Files (x86)\Steam by default.
Be sure to close Steam completely (click the Steam menu at the top of the Steam window and select Exit) before deleting this file.
You’ll have to restart Steam after changing this setting.
If none of these solutions helps, try Googling the problem – you may have run into a game-specific problem with a game-specific solution or just a more obscure Steam bug.
If a game fails to load, crashes as soon as it loads, or crashes when you reach a specific level, your game’s files may be corrupted.