If your Apple Watch is not responding, and you can’t turn it off or on, you can try forcing it to restart.
To force restart your Apple Watch, press and hold both the Side button (on the right side of your device) and the Crown button (on the right side of your device) at the same time for at least 10 seconds. When the Apple logo appears, release both buttons.
Please never force-restart your Watch if you’re updating watchOS. If you see the Apple logo and progress wheel on your Watch screen, leave your Apple Watch alone and on its charger until the update has finished.