Force-Restarting Your Apple Watch
If your device is seriously frozen and won’t respond to the side button action, then press and hold both the side button and digital crown at the same time for about ten seconds. The screen will power down, then the Apple logo will be displayed, in which case you can let go of the buttons.
However, if you’re updating watchOS on your Apple Watch, don’t attempt to force-restart it. Just wait it out if it feels like it’s taking too long. You should see the Apple logo and a progress bar during an update.