first off i am no expert just a guy who wants to help so dont be supprised if what im saying doesn’t help but here goes. i would suggest to try and locate the PCB for the display and see if either display might have shorted it. if there is no of this then most likely the ribbon connector that is at fault considering an external display still works and considering that a replacement ribbon connector is a whole lot cheaper than a logic board replacement. if nothing works just look and see if there is anything which looks like it doesn’t belong and you might find something.
good luck
-M