|The power supply should shut off if you push down and hold the on/off switch for at least 4 seconds.|
Could someone have messed with it? It the power supply has a 115/230 switch on the back make sure it's in the correct position. DON'T GUESS and don't switch it while it's plugged in.
You can try removing all the cards except video and disconnect all the drives. (Make notes about reinstalling them, if necessary.) Then start it up again. If you get a posting screen then one of the removed items is at fault.
But likely it's either the motherboard, power supply or both.
A power supply can turn on but still be bad. It needs to supply several different voltages at sufficient amperage. So while it may provide the 12 volts for the fans that doesn't mean it's supplying the other power requirements.
And if a power supply goes bad it can often ruin the motherboard as well.
It it's not one of the attached peripherals you can try swapping the power supply with a compatible known-working one. If it still doesn't give a posting screen it's almost certainly the motherboard.
DAVE says, "Don't be an idiot. Vote Republican"