Hi aideen160,
Thanks for posting in Apple Communities.
With Apple Watch logo onscreen, you want to force restart it.
Don’t force restart your Apple Watch if you're updating watchOS. After you see the Apple logo and progress wheel, leave your watch alone and on its charger until the update finishes.
To force restart your Apple Watch, 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.
You can find these steps in Restart your Apple Watch.
If you're unable to power it on, contact Apple Support for service options or find a nearby Apple Authorized Service Provider.
Regards.