Apple Watch series 8 black screen

Does it still show the charging icon when you place it on the charger?

If your Apple Watch won't charge or it won't turn on - Apple Support

Did you already force restart your watch?

To do that, press and hold the Digital Crown and the Side Button together for about 10sec, until the Apple logo comes back again.

Restart Apple Watch - Apple Support

It may take up to 20sec, until the Apple logo comes back.

You can try this while the watch is on the charger or without being charged.

If this does not work, you should get the Apple Watch looked at or serviced:

Apple Watch Service and Repair - Apple Support