Diagnosis
The membrane switch for the down button inside the handset is faulty or the wire connecting it is damaged. Moisture or physical damage is the usual cause.
How to Fix It
You need a new handset. The part is usually called the "keypad" or "controller." It connects via a thin 6-pin telephone-style cable to the control box. Order the replacement, unplug the old one, and plug in the new.
If That Didn't Work
The issue could be in the control box's input circuit. Try a known-good handset first to confirm. If a new handset doesn't work, the control box PCB has failed and must be replaced.
Heads Up
Do not try to open and repair the handset. It is sealed and not user-serviceable. Spilling liquid on it will almost always cause this failure.