I had the same exact problem with a gateway gt5670 desktop. I replaced the motherboard and installed everything and the cpu fan and system fan would spin but no video or LED light. By the way it was my brothers pc I was working on. Tried an external monitor and it detected nothing. So i took out the cpu and found it had a bent pin. Replaced it with and known working cpu and the pc powered and posted. Using it now. Hope this helps.