PC momentarilly starts then shuts down

When I turn on the PC all of the fans and LEDs light up momentarilly then everything shuts down.I have a PC with an ASUS PSN32-E SLI plus installed. I reseated all of the connectors on the mobo, HDD and Opt drive. Then I unplugged on cable at a time and restarted after each. When I removed the ATX12V supply cable from the mobo it started and stayed running but I received no video out. This cable is labelled P2 and is 2 black wires and 2 yellow/black striped wires. I think this is the CPU power? Anyway I don’t know if this is a mobo problem or if I can replace something else? I didn’t re-seat the CPU as this appears to be clamped into place.
Thanks,
Jim