External wiring connects the wall-mounted door control and the safety sensor to the motor unit of the garage door opener. That wiring can become damaged or disconnected. Check that wires are securely attached to the sensor's photoelectric eyes (each eye has 2 wires) and the wall unit. If any wire is cut, strip insulation from the end of the wire and reattach it. Check that wires are still attached to the terminals on the back of the motor unit. Replace or repair any frayed, pinched, or exposed wires.