not sure if you have a furby with a tongue but...according to the Furby hospital this procedure may work.
hold down tongue
turn furby upside down
shake furby a bit
turn right side up and release tongue.
You may have to hold tongue longer or try again. Just be gentle.
The only thing close to a motion detector circuit would be the Light sensor photocell in Furby's forehead. I would start on that one and see if it is still functioning. For way more info on the Furby, check this site. It has great information and plenty of autopsy pics. Attached is the diagram for the Furby's electronics. Hope this helps, good luck.