I had the same problem and found no answers on line from the "experts". I tried different things but what finally worked is holding power & back buttons in at the same time for about 8 seconds, then watch rebooted & problem solved! I deleted the recent watch faces I had downloaded & turned off the always on feature. Hopefully problem permanently solved......
I have the same problem. Forcing a reboot with the 2 buttons immediately shows the "rebooting..." screen.
Nothing else wakes up the screen, but the watch is connected to my phone and the watch resonds with vibrations to notifications.
Only standard watchfaces; and always on activated.
If it happens again, I would try to perform a factory data reset. And if it happens again after you take that step, I would reach out to 1 800 726 7864, our Samsung+ app, or to one of the moderators here to schedule a repair for that device.