Did you perhaps accidentally mess around with any of the files in Amnesia's root? This error does come up sometimes when you start the game manually.
The first place to start would be to perform a Steam File Integrity Check to see if all the necessary files are still working alright. Check
Problem I of the
Troubleshooting Guide if you require some assistance with that.
Otherwise, what Operating System are you using? Windows allows DRM-Free use of Amnesia versions greater than v1.3, but I believe Mac does not (Mudbill could probably help there), and I'm not sure at all of Linux. Did you move all of Amnesia's files to another directory?