Did the display just stop working?
Does it cycle through the high scores or spell out Dr. Dude? Does it show an error message (batteries died/not good connection), Does it function properly in test/adjustment screens? Does it function properly during gameplay? You said it has other activity but is it at random and does it make sense?
If the display is showing all zeros and the segments look bright and are full then that rules out quite a bit. Your probably not looking at a power issue (100vdc + and 100vdc-), its probably not a short or a bad cap, transistor, exc.
I would be looking at the ribbon cables and confirm there is good connection and are properly plugged in. I also would confirm the slam tilt (if installed) is not closed.
Try swapping displays. (power off. Careful HV)
Maybe a quick vid of the display upon boot up would be good too.
Thanks
Blake