Originally Posted by digit 
You tried shorting the pins to see if that results in a power on? You could in theory short all pins and you'd eventually get the right ones
Its possible I guess the wires to the terminals on your power button might have a bit of a dodgy connection or come off. If possible check to see if the wires lead directly to the power button and that they are connected, perhaps fiddle with them a bit just to make sure they are connected.
I've tried to short out all the combinations of the pins present. I get a few sparks when i short out a specific pin, and when doing the so the LED on the motherboard turns off, until you stop shorting it.
I'm at a loss at what to do
EDIT: I can't access the button on the case, very difficult, and i can't remove it, to take a closer look