Break it down to a bare bones boot...remove everything from the motherboard, including the RAM and try booting. It should beep at you. If it does, add things back one at a time...starting with the RAM, then video card (if you have onboard video, ignore that). If nothing happens, try a different power supply. If, after swapping power supplies, nothing happens, search the board for bulging or leaking capacitors (if you have good access to the board you can do this as step one).