MoBo Power issue

I am working on a client’s PC and am in the process of replacing the motherboard. All connections (to the best of my knowledge) are connected and when I press the power button, all the fans go on and it sounds like its on. Nothing else happens though, the harddrive isn’t spinning, the video doesnt display. Its just the fans that turn on and thats it. What could cause a problem like this? I want ALL possible problems. So far this is what I’ve come up with.

-weak power supply
-screws maybe?
-bad MoBo
-jumpers?

I don’t know. I can’t think of anything else. Does anyone have any neat tricks I can do to maybe get this system to work?