mercy

OK, guys, I'm new at this and desperately need help!! Computer was working fine one day and I get a blank monitor the next day!! I've put in a new power supply and power cord. I checked the monitor on another computer and it works. I also tried the video card and HD on another computer and they both work fine. So, monitor, HD and video card are all OK. I've doubled checked that the memory is in right and that all ribbons are connected well. I'm actually down to either the AGP slot went bad (can they do that with no warning at all??) or something is wrong with the motherboard. The reason I mention the AGP slot is that I took the video card from my computer (the one I'm working on right now) and tried it on the problem computer and I get nothing. So, next question is how do I determine if it's the AGP slot or the motherboard? I'm not too computer savvy so the next statement may sound dumb (sorry!) but FYI the motherboard's little green light is on (if that means anything) and when I turn the power on the CPU green & red lights come on and so does the DVD light. Not sure if those being on has anything to do with the motherboard. Sorry if I can't explain myself better. The computer worked fine one day and just decided not to go on the next with no warning or beeps whatsoever so I am completely lost. Thanks for any help or suggestions. The motherboard is an ASUS P4SDX.