How to restart your Apple Watch

1. Press and hold the side button until you see the Power Off slider.
2. Drag the Power Off slider.
3. After your watch turns off, press and hold the side button again until you see the Apple logo.

If your Apple Watch is charging, take it off the charger first. When your watch is charging, you can't restart it. 

Don’t force restart your Apple Watch if you're updating watchOS. After you see the Apple logo and progress wheel on your Apple Watch, leave your Apple Watch alone until the update finishes.

You should force restart your device as a last resort and only if it's not responding. To force restart your Apple Watch, press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo.