You've tried resetting, unpairing, and re-pairing. It's not in Theater Mode... OK. How about the force restart that fieryjack suggested?
Question, when the screen is on, it's on, right? Explain what it looks like when it's "Not Working"? Is the screen completely black, can't see anything?
If you can see the screen, but it's not as bright, this is how it's designed to work. Your battery would die rapidly if the screen stayed bright all day.
*** Screen is black ***
I doubt that it's this simple, but check this:
From your watch, go into Settings, > Display & Brightness, > Always On.
Is the slider green? It needs to be, toggle it on.
You can also get to that setting using your phone.
Open the Watch app. Click on Display & Brightness, Always On (or just read its status), is the slider green? It needs to be, toggle it on.
If it is currently set to "On" and it's not working (black screen), toggle it off, reboot your watch, go back in there and turn it back on. Does it work now?
Let us know ;-)