Could you have Screen Curtain enabled? See the ;'Use Screen Curtain on your Apple Watch' sec ion in:
Use Screen Curtain with VoiceOver for your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support
The first (and simplest) thing to try if something isn’t working right is to restart your Apple Watch and its paired iPhone. If necessary (device not responding), try a forced restart. Both methods can be found here:
How to restart your Apple Watch - Apple Support
Restart your iPhone - Apple Support
This could be a problem if the screen doesn't work, but if that doesn't help, the next steps would be to unpair and pair your watch again and restore from the backup just created. This should result in no loss of data. It deletes all temp and corrupted files and re-indexes the file system.
Unpair and erase your Apple Watch - Apple Support
Set up your Apple Watch - Apple Support
I suggest you make an appointment at the Genius Bar in an Apple Store near you and let them look at it:
Genius Bar Reservation and Apple Support Options - Apple
Your Apple Watch is water resistant, but not waterproof.= For example, you may wear and use your Apple Watch during exercise (exposure to sweat is OK), in the rain, and while washing your hands. For more info, see:
About Apple Watch water resistance - Apple Support
If water splashes on to your Apple Watch, wipe it off with a nonabrasive, lint-free cloth. Don't use heat, compressed air, or sprays. Clean and dry your Apple Watch, the band, and your skin after workouts or heavy sweating. After swimming, gently rinse Apple Watch Series 2 and newer with warm tap water. Dry Apple Watch and the band thoroughly if they're exposed to fresh water. Learn more about cleaning your Apple Watch.
