There is a proximity sensor on the front of your watch that allows the watch to know when to sleep. Try giving the face of the watch a wipe with a clean, dry cloth to make that there's no moisture or dirt on the screen.
Also, have you checked that there's no damage to the screen of the watch?
It may be worth restarting your watch by pressing and holding the Home and Back buttons > Select Power off > After a few seconds, turn it back on by pressing and holding the Home button.
Hope this helps 😊.