Assuming the pickup/display unit is similar to what's on a PM3520B, it is likely that the pickup disk mounted on the spindle has shifted to the side so that the pickup diode no longer sees the disk. If this is the case, moving the disk back to its original position should restore normal operation.
Remove four screws (at the corners) and pull the display unit out of the headstock case until you can see inside. You should see the disk, which is secured to the spindle with a setscrew (or maybe two).
The pickup diode mounted on the display unit should line up with the disk. Here's a photo of the diode on a PM:
This shows the relationship between the pickup and the disk on a PM:
Good luck! ~Doc