Are you not getting the POST screen at all?

Try pulling all expansion cards out, and removing all the cables. Only have power going to the motherboard. Unplug everything from the MB and have only a video card and memory on the board. This is the bare minimum to be able to see a POST screen. If this won't come up then you've reduced the problem to, MB, CPU, PS, Video, Case. If it does, then re-add one component at a time.