Force a boot while still on the charger

1. If the screen is still completely black, clean the magnetic contacts on the watch and the charger, connect it to a low-power source, and leave it plugged in undisturbed for 30–60 minutes.
2. While it's still attached to the charger, press and hold the power/SEL button for 30–40 continuous seconds — longer than a normal restart press. This can force a hard restart of the power-control chip if the watch is stuck in a deep discharge state.