Adaseb what if the GPU don't show any signs of something got blown ? What if the fuse aren't blown and no capacitor got damaged and yet the GPU fan only spins with no display? Is there something else to look into?
Put it in directly into the motherboards x16 slot and use a bootable DOS disk and run atiwinflash and see if it can detect it. I had a GPU that did this before and all it needed was a reflash to start working again.
However I never connected it backwards on the riser, usually from what I read about people who did that is that the GPU is toast because there is a pin in there thats an actual +3V or +5V and you fed that load to somewhere which was only meant to be used a signal pin.
Try and see if it boots in DOS first.