Work the problem down some. First try putting the motherboard on a static bag and put the CPU and memory in it, attach a power supply and see if it works (I am assuming it has on-board video, if not put a video card on it). If that does not work then start replacing components, you can't assume anything works until you verify it. If it works then start adding components and finally put it in the case. It may be a slow process but it works most of the time and it is the best way to find the problem.