Yeah, this one's common. Either the switch actuator is jammed or the drive motor wire is disconnected.
First, check that the switch actuator (part number 3129714001 or 3131603001) on the control handle moves freely. If it's stuck, clear any debris. Next, trace the internal wire assembly (part number 2826843001) running to the self-propelled unit. Make sure it's plugged in securely at both ends.
If that didn't work, the switch electric assembly itself (part number 2830923001) might be faulty. You'll need to test it for continuity.
Heads up: Before digging into the wiring, make sure the battery is removed. It's a 56V system—you don't want to short anything.