Video Card installed correctly? RAM bad? Motherboard mounting screw in wrong spot (seriously, i've done it before and had the same thing happen)?
Try taking your motherboard out of the case and placing it on a wooden table. Hook everything up to it (CPU, video card, heatsink, RAM, PSU, and power button) and turn the machine on. If it still refuses to POST, then I would say either bad RAM, motherboard, or video card. Just be sure the motherboard is on a wooden table when you do this, placing it on the wrong material and starting it up can fry your motherboard.