Apple Watch's screen is black even though its fully charged

First press the side button or the digital crown to see if the face will come on. If the face will come on with that, most likely theatre mode is turned on. Turn off theatre mode via the mask icon in the control center

If the watch is still not coming on you might want to force restart it:

Press and hold both the side button and Digital Crown until the screen turns black and the Apple logo reappears, then release both buttons. Your watch may take a few moments to restart. Never do this when updating watchOS

If the Apple Watch is still not working you might need to set up service for it. Please Contact - Official Apple Support to do so