Sounds to me that the memory expansion board inside got loose. Give it a while to rest and then try to turn on again. Opening the Duo (any Duo as they all share the same case) is a lot of trouble. You can just remove the keyboard with the 4 screws at the bottom of the case that lines up horizontally and the keyboard lifts up. There are two ribbon connectors holding the keyboard ribbon cable on the middle left by the palm rest.

The memory card is at the middle right of the board if you have one, the modem card is above that on the upper right if you have one, if not there will be a blank card for the reset button. Just push the memory card back into the connector slot.

This is a link for the service manual. http://tim.id.au/laptops/apple/powerbook/powerbook_200_series.pdf