|Did you also disconnect all external hardware? Try booting with minimal hardware. |
MBoard, power supply, CPU/HS/fan, 1 stick of RAM, video card, keyboard, monitor. Disconnect EVERYTHING else. No drives, no additional cards, no case wiring, no bracket wiring, no card readers, etc. To start the system momentarily short the two pins that normally connect to the power switch.
If no go then you may have a dead CPU of motherboard. I am assuming the RAM is good. You could try a different stick to be sure.