there is a proximity sensor located in the base of the handle arm at the drive unit where the control arm pivots, it may or 'may not' click.
you would have to disconnect it from the harness and check it with a vom meter to verify if it is working or not.
This switch/sensor interfaces directly with the motor controller and has to operate to turn on the trucks systems usually via a main contactor.
When you turn on the truck and pull the handle down this should activate turning on the rest of the systems, if it is working normally you would hear the main contactor pull in (click).

This unit should have a battery discharge meter display unit, look at the hour meter display and see if it is showing any code when you try and activate the truck, pull the handle down and try to raise or move the unit and watch the display. Is it showing a code?
If this unit has no meter display then you will need to check the led on the motor controller and see if it is flashing any codes. This unit should have the battery display unit on it so you should see an error code displayed on it and also see a red led flashing on the motor controller if any problem exists.

plus make sure battery voltage is good and you check the main power fuse and the control fuse(s) on the panel and even the one on the battery charger. (yes the battery charger interfaces with the truck too and might cause issues if that fuse is blown)

good luck