Press and hold the Side Button until you see the Power Off slider. Slide it to the right and the Watch shuts down. Press and hold the Side Button again until you see the Apple logo. The Watch will restart.
If that doesn't fix the issue, try a Force Restart. Press and hold both the Digital Crown and Side Button. The screen will go dark for several seconds. Release the Crown and Button when you see the Apple logo. The Watch will restart.
An Apple Genius told me that Force Restarts should be a last resort, because they wear on the firmware a little, each time. I assume it can cause directory damage, but I didn't know about firmware. I don't know if a new wOS can be installed, as with macOSs.
04-16-2017 06:28 AM