Apple Watch series 1
My Apple Watch will charge but if it turns on it will stop working almost immediately or won’t turn on at all. It is clearly charging correctly but something else is wrong.
Apple Watch Series 1, watchOS 6
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You can try these steps:
If your Apple Watch won't charge or it won't turn on - Apple Support
but it just may be time to get the battery replaced (or get a new watch). For repair & service (including battery replacement0:
watch Repair - Official Apple Support
watch Service Pricing - Apple Support