Apple Watch won’t turn on

First try to Force restart the watch by pressing the side button and Digital Crown button at the same time until the apple logo appears, then release both buttons.

See: If your Apple Watch won't charge or it won't turn on

“Apple Watch won’t turn on: No matter how long I charge or what I do, I can’t get my Apple Watch to turn on all of a sudden. Any help would be greatly appreciated!”

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Go Here: Get Help if your Apple Watch won’t Charge or Turn On - Apple