My notes on the non-Fanuc stuff are at home. I believe the PBU LED is connected to the DC servo voltage (90VDC?) that comes from the servo transformer and through the rectifier. There's a fuse somewhere, which is probably blown. It will be a pretty big fuse in a separate holder as I recall.
Have a meter? You can follow the wires to the servo transformer which is the biggest transformer if there's more than 1 transformer. The output should have AC voltage, probably around 75 to 100 VAC. Follow those wires and you'll find the fuse & rectifier. The output of the rectifier should be about 90VDC or perhaps 180VDC depending on the servos.
It's not surprising that it blew, as the Z was trying to push the tool without the spindle turning.