Shout out to Chantelle - changing the clock face in the app worked for me too. Screen was black on charger and off charger. It was still syncing with the app so that was working. Tried force restart and got nothing- held the two buttons while it was on the charger for a full minute and nothing. Confirmed it wasn’t in water lock mode by double tapping the screen a dozen times.
Changing the clock face in the app- boom screen is back and buttons working again. Kinda stupid but hopefully this helps someone.