You are going to have to fault find it, I would start with checking power is getting all the way to the neck section. if you are lucky it might be something silly like a bad connection, looking at those little spring loaded connectors on the top of the pack (body section) im guessing those have matching connection strips on the bottom of the neck section where the main pcb lives if one of those is stuck or the spring has melted or something it might not be connecting well with the neck section , if you can get the front of it check to see if you actually have power coming in to the pcb with the screen.
Looks like you are not the first to have problems with this unit
Imalent DX80 Gen2