Brightness on Apple Watch
My wife has a new series 5 watch. Over night the brightness is so low that I can’t read or see the display. I ve tried settings/brightness but unresponsive. Screen is virtually black.
If you followed the procedure in:
Adjust brightness, text size, sounds, and haptics on Apple Watch - Apple Support
and it still doesn't help, then I would use the "Contact Support" link at the top-right corner of this page (especially if the watch is still under warranty).