Hi Pete, how are you? Greetings from Argentina.
Your Jungheinrich forklift was misconfigured the type of machine, perhaps it had too much voltage drop due to using the equipment with low battery levels.
There are two ways to make your machine operational again through Jungheinrich software or through setting the machine type from buttons on your display.

I hope my answer has been helpful.

Greetings Ironman_93