It rarely happens (at least to me), but if your Apple Watch freezes on you, you can force restart it. However, first try to restart it and its paired Apple Watch.
To restart the Apple Watch (make sure it’s not charging)
- Turn off your Apple Watch: Press and hold the side button until the sliders appear, then drag the Power Off slider to the right.
- Turn on your Apple Watch: Hold down the side button until the Apple logo appears.
To restart the paired iPhone
- Turn off your iPhone: For models with Face ID, press and hold the side button and a volume button, then drag the slider to the right. For models without Face ID, press and hold the side or top button until the slider appears, then drag the slider to the right. With any model, you can also go to Settings > General > Shut Down.
- Turn on your iPhone: Hold down the side or top button until the Apple logo appears.
Force Apple Watch to restart
If you can’t turn off your Apple Watch or if the problem continues, you may need to force your Apple Watch to restart. Do this only if you’re unable to restart your Apple Watch.
To force restart, hold down the side button and the Digital Crown at the same time for at least ten seconds, until the Apple logo appears.
The accompanying graphic is courtesy of GottaBeMobile.
(This how-to is based on my experiences and info on Apple’s support pages — where the images sometimes come from.)