How do I re-install Amnesia purchased off of retail?

[Romulator]

Since it was a Humble Bundle purchase, which is okay, you have either one of two things, a DRM-free copy or a Steam copy.

With the DRM-free copy, you need to revisit the Humble Bundle website and log in to a personal account there and reinstall Amnesia that way. If you have not used one to purchase Amnesia, you'll need to navigate back through your mail and find the store link to the bundle or Amnesia itself through that. Searching for Humble Bundle and finding something along the lines of "Thankyou for your purchase!" will help in locating the link to the Amnesia bundle you purchased. From there, you should be able to redownload the installer and apply it.

If you are using a Steam copy, a simple reverification of files may work. To do so, right click Amnesia in Steam, then select Properties. A window will pop up, and by selecting the "Local Files" tab, you can select "Verify integrity of game cache". This is much less painful than a reinstall and will generally fix most problems if that is the case.