Black screen in my Apple Series 6 watch
I’ve tried holding the side button and dime for 15 seconds without success.
If you have tried all suggestions mentioned in this related article, and the watch still does not turn on, you will have to contact the official Apple Support to get it serviced.
If your Apple Watch won't charge or it won't turn on - Apple Support
You can also visit an Apple Store/Authorized Apple Service Provider next to your place to get it looked at.