Black screen on my watch

Perhaps this article will help:

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

If you still need help, contact Apple Support.

It might be a hardware problem. For repair & service:

watch Repair - Official Apple Support

watch Service Pricing - Apple Support

Does watch appear if you press button? Make sure you don’t have theatre mode turned on. You can find that by swiping up.