Well if the boost inverter that provides the voltage isnt running, thats why. I dont have a PB100 in front of me, but i know on quite a few powerbooks there are pico fuses around the board. and one of them blown in the boost inverter circuit would cause it. You could also have a shorted transistor instead as well. Time to find out where the boost inverter circuit is, and check it out.

Then again, Im not sure.