Apple Watch
My watch would not power up and charge and it’s overheating on the charger
Apple Watch Series 4, watchOS 7
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Perhaps this article will help:
If your Apple Watch won't charge or it won't turn on - Apple Support
but if it is very hot, kt may be a hardware problem with the battery. For repair & service:
watch Repair - Official Apple Support
watch Service Pricing - Apple Support
If you still need help, contact Apple Support.