Black screen possible fixes;
A common reason why an Apple Watch face becomes black and unresponsive is that the Watch isn’t charged. Try placing your Apple Watch on its charger to see if it comes back to life.
Simultaneously press and hold the digital crown and side button until the Apple logo appears on the screen. You may have to hold both buttons for 20–30 seconds before the Apple logo appears, so be patient.
Open the Watch app on your iPhone and tap Accessibility/VoiceOver.
Make sure the switch next to VoiceOver is off. Then, scroll down the page and make sure the switch next to Screen Curtain is off.
And as a last resort unpair and repair the watch and setup as new not from a backup.