My Apple Watch with a full charge is not working. What is going on?

First force restart the watch. i.e. press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo. 

Then force Restart the iphone  

Force restart iPhone – Apple Support (UK) 

If this does not trigger it into life come back, or reach out to Apple Support directly. You should be able to reach them here

Click here > Contact - Official Apple Support

or phone numbers

Click here > Contact Apple for support and service - Apple Support

and or

Genius Bar Reservation and Apple Support Options - Apple