What do you mean by "won't work" ? Need more details.
Any lights? Any fans spin? Does it light up / fans spin but just no display on the monitor?
If it's not doing anything, triple-check all of your connections. Reseat the RAM/video card (if you have one). If that doesn't do it, remove the motherboard from the case and put it on a piece of cardboard, and then short the 2 pins for the case's power switch together with something like a screw driver to see if you can get it to turn on.
Also, please post your specs so we know what we're working with.