How to hard reset an apple watch, series 9
I have a series 9 watch that stays with a black screen. I have had limited success with the hard reset, which gets the watch working, though it locks up again and goes to the black screen.
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If the force restart did not work, unpairing the watch would be the next step:
Unpair and erase your Apple Watch - Apple Support
In case this does not work either, it looks like a hardware issue, and the watch needs to be at least diagnosed by Apple or an Authorised Apple Service Provider:
Apple Watch Service and Repair - Apple Support