Could be any number of things. Why not list your system specs to make things a little easier, including the brand of the power supply. 400w is meaningless by itself, a quality 350w PSU is superior to a generic 400w PSU, and the rating of the 12v rail is very important for modern systems. Is this a new build, or something that was up and running, and suddenly stopped working?
Several things I have seen that cause your exact symptoms:
CPU not fully seated
No fan plugged in to CPU fan header
Motherboard not mounted properly on standoffs(ie a standoff is out of place shorting to the motherboard, screwed motherboard directly to case without using standoffs, loose screw under motherboard).
Bad power supply
Try booting with the bare minimums. 1 stick of ram, video card, CPU/Heatsink and Fan. If that doesn't work, try with the bare minimums, but remove the motherboard from the case, and put it on a non-conductive surface.