Please check the troubleshooting guides before posting!

[MuppetNeko]

THANK YOU, THANK YOU, THANK YOU! It works like a charm now.
A lil' bit off topic but is there any way to lock the frame rate to a certain number?

[plutomaniac]

So resetting the Nvidia control panel fixed it, right?

A specific FPS number, no. But you can enable vSync which will do that at fixed values of 30, 60 etc...

[MuppetNeko]

I choose the Nvidia card to be default for everything when playing Amnesia, otherwise it simply doesn't show. Also thanks for the fps tip. ^^