BAC855 with NXT display

At a guess, it isn't able to communicate with the controller, and that is usually a connection fault between the two.

What happened to the system between the time it worked and the time it did not?
 
A connection problem can happen anywhere from the electronics inside the controller all the way to the electronics inside the display. Unfortunately the only "easy" way to test that is to do a continuity test with both parts open (but unpowered, no battery connected) to access the solder pads the wires go to, and use a multimeter on ohms or continuity.

Alternately, multimeter on DC volts can be used to test for signal on both TX and RX (if it has those, and not some other communications format/connection), where you should see about 2.5v (the average of the 0-5v pulses on the wires) on each one. Whichever wire doesn't have that is the problem.

If both have it at both ends, then the problem is likely the electronics within the display or the controller, unable to receive or to send, or both, due to some failure (probably a broken solder joint, or failed layer interconnect (barrel) on a PCB, sometimes an actual component failure).

If the display housing is 3D printed as it appears to be, it may not have a good watertight seal and moisture ingress could have caused corrosion.
 
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