"Trying to power off" may need describing.
It may be drawing towards end of life.
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, although it may seem irrelevant, Force Restart the paired iphone Force restart iPhone – Apple Support (UK)
If is is still misbehaving...
Unpair and erase your Apple Watch
and then
Set up your Apple Watch
If no better you may want to unpair then set up without restoring the backup.
You may also want to make sure the software on the watch is up to date.
Update your Apple Watch
If you've completed that troubleshooting and the issue is still not resolved, Contact Support.
Click here > Contact - Official Apple Support
or phone numbers
Click here > Contact Apple for support and service - Apple Support