The unit sits on its own custom waist basket. In the unit is a micro switch that must be clicked by the seating of the unit on top of this waist basket. If the micro switch isn't activated, forward won't work. Check this. Short of this, the microprocessor that runs the diodes might not be working. (The diodes, one transmitter, one receiver, tell the machine the presence of a piece of paper in the slot.)

After seeing the other suggestion I got a can of compressed air from a computer store to clear the sensors in the feed slot. It needed a long blast but it started working again. My instructions say, every 6 months, put a few drops of olive oil on a sheet and feed thru; try every couple of months to avoid the sensors getting covered in dust etc