Watch : black screen: It could be the charger not giving power :
The restart was a good first step:
Apple support have an article online that may help you check whether the watch is charging:
( If not: See if you can check whether the charger, and the power point are both working ok. )
If your Apple Watch won't charge or it won't turn on - Apple Support
About: Apple Watch Battery and Performance - Apple Support
If necessary: Get service for your Apple Watch battery - Apple Support
or
You might want to reach out to Apple Watch Support.
There is a "Get Support" link (scroll to the very bottom) on their webpage (link below)
Apple Watch - Official Apple Support
All the best :-)