Well, darn. Yesterday I awoke to find my Apple Watch unresponsive. Nothing that I tried revived it, so I resorted to a force restart. If you need to do this, press and hold the side button and the digital crown until it restarts.
Forcing your Apple Watch to restart won't erase information or return your device to factory settings. And, as Apple notes, you should force restart your device as a last resort and only if it's not responding.