While the Apple Watch is a great device, let’s face it, it’s really just a tiny computer, and someday you’ll likely need to restart it, just like you do your Mac, iPhone or iPad.
If your Apple Watch ever becomes responsive due to an app, or a runaway process thread, you may need to restart it to make things right. If you’re an iOS user, the method will likely be quite familiar to you.
Try Turning Off and Then Back On
First, try turning it off, and then back on again. To do this, simply press and hold the side button until the sliders appear on the screen, then drag the Power Off slider to the right. To turn Apple Watch back on, hold down the side button until the Apple logo appears.
What if That Doesn’t Work?
If that doesn’t work, you can force restart the Apple Watch. Just hold down the side button, and the Digital Crown at the same time until you see the Apple logo appear. (It should take about 10 seconds.) Release the buttons, and your Apple Watch will restart.
(Apple Watch Help Website, via Cult of Mac)