Amnesia: "Failed to start game (app already running)"

[TheBlastPassage]

This is a Steam-related issue. Restarting Steam and/or your computer may resolve this issue.
Sometimes, a re-install of Steam is required. I would recommend that you backup your Steam games before re-installing Steam. To do this, just copy the 'steamapps' folder in your Steam install directory to your desktop and then put it back in your install folder after you have re-installed Steam.

Here is where you can find your steamapps folder: (just open up the spoiler)

Spoiler below!

32-Bit Windows: C:\Program Files\Steam
64-Bit Windows: C:\Program Files (x86)\Steam

Hope that helps your problem. If not, please feel free to let me know so I can continue to help you