Perplexing Problem

[fcchief]

I am trying to build up a computer for my mom with my old ECS K7S5AS Mobo, hard drive and ATI video card. I installed the mobo and power supply in an old case I had. Problem is when I hit the power button nothing happens. I disconnected all LED and the restart leads from the FBI and left the pwr switch hooked up, still nothing. I then took another pwr supply I had laying around and hooked that to the mobo, grounding it to the case and again no power. I am perplexed, the motherboard, and hard drive worked fine just 3 weeks ago prior to upgrading my computer. Any suggestions I would be happy to hear them.