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, while it is restarting Force Restart the paired iphone Force restart iPhone – Apple Support (UK)
Test it again. If it is still misbehaving... Unpair and set up again
Unpair and erase your Apple Watch – Apple Support (UK)
and then
Set up your Apple Watch – Apple Support (UK)
If no better you may want to unpair then set up without restoring the backup.