if u have no POST, IE the keyboard lights don't blink and there is no HD activity. my fist guess is a CPU failure.
It could be power, but if ur lights on ur optical drives are flashing I suspect power is OK.
If u have a meter u can verify them, but as I said, the drives will do their own POST independent of the system as long as there is power. That seems to be happening by what u say. If the CPU is dead nothing else will happen, no POST, no kbd lights, no hard drive activity - except for its initial self test.
if none of the drives do a self test as soon as u power up, then the 12v might be out because the 5v powers the lights on them.
A CPU failure might be caused by a heat sink failure, but it should attempt to start when it is cold if that were the case.
good luck