Jump start the motherboard. If your metal cover over the box with the boards has a label that says "Reset Switch Inside", dig around and find it. They hide it so you don't push it accidentally.
If you don't have the label, then you'll need to briefly short the two pins on the motherboard a computer start button would plug onto.
That should do it.
Email me if you'd like pictures where to short the two pins. service AT jerseyjackpinball DOT com.
LTG : )