https://support.garmin.com/en-US/?faq=LePCURziXd4A5qZLwqKEwA
A watch that is not responding can be recovered by attempting the following steps.
NOTE: These steps may result in a loss of some data such as recent steps taken or a loss of current time. To correct the time see: My Garmin® Watch Displays the Incorrect Time
For watches with a physical button, press and hold the power button until the watch powers off (hold up to 30 seconds).
Release the power button.
Press the power button again to turn the watch back on.
If your watch is rebooting, ensure you have applied all available updates to your watch. Available updates can be installed by using the Garmin Express™ program.
For additional troubleshooting steps on this issue see:
My Garmin Watch or Edge Device Will Not Turn On
My Approach Watch Will Not Power On or Charge