Sounds like the screen may have died.
You haven't tried to remove the screen have you? https://www.youtube.com/watch?v=OpwpN1lORMo&t=1s
I showed how easy it is to damage the cable in the process of repairing a core.
I had it open to check for any sign of damage/bad contact but couldn't find anything,
@ajb2k3 tday I've loaded one of old firmwares with old M5burner and it is same...only LCD backlight is on all the time not a single thing being displayed on LCD, if I use ERASE option then backlight is off