Ademco Keypad not working after extended power outage

I have a Ademco Vista 20SE system with two keypads (M6673). Following a
winter power outage of 4 days, one of the two keypads doesn't work,
however the system works fine with the other keypad.

I verified that the keypad is the issue by switching the two keypads
(dismount from wall and swap) so I am confident that all wiring is
fine.

On power up, the bad keypad turns on all LCD indications, lights both
the LEDS (red Green) and then blanks the LCD display. The keypad
responds by beeping o ..
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