Screen is black
My watch is powered on but screen is black
Apple Watch Series 3, watchOS 7
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Best reply
From:
If your Apple Watch won't charge or it won't turn on - Apple Support
"If your screen is blank or you see the Apple Magnetic Charging Cable icon and a red lightning bolt 
on the screen, charge your watch for up to 30 minutes."
If that doesn't help, it may be a hardware problem. For repair & service:
watch Repair - Official Apple Support
watch Service Pricing - Apple Support
If you still need help, contact Apple Support.