Summarize the following:
Raise your Apple Watch wrist, or press either of the Apple Watch's buttons. Doing so gives the Apple Watch enough time to finish turning off. This will turn back on your Apple Watch; you should see the Apple logo appear after a second or so, after which point your Apple Watch should be fully on within 30 seconds.  You'll need to enter your Apple Watch's passcode after the Apple Watch finishes restarting. If the Apple logo doesn't appear within ten seconds of pressing the Power button, press it again.

Summary:
Wake up your Apple Watch's display. Wait ten seconds. Press the Power button.