Let’s face it: A time will come when your Apple Watch crashes or freezes. It’s gonna become unresponsive, but you’ll be ready thanks to this handy Apple Watch restart tip.
When your Apple Watch freezes, it’s likely due to some app or another that kills it. At this point, you’ll want to force the lovely digital gadget that you wear on your wrist to shut down and start up again.
The way to fix it will be familiar to most iOS and OS X users: You’re going to have to force the Apple Watch to restart.
To make this happen, all you need to do is hold down the side button and the Digital Crown simultaneously for at least 10 seconds. Then you’ll see the Apple logo appear.
If you’ve had to force your iPhone to reboot, you’ll know what that’s going to feel like.
Source: Apple Help