I have had this watch for 4 months and was fine. Just this week, twice, pulled from charger in the morning, received the "Good Morning", then a bit later, black. Tried holding every combination of buttons for over 8 sec, no go. Put it back in the charger, and it worked. This morning, same black screen, and put it back in the charger, that did not work. So, back to holding the buttons for long time. It took 3 tries, and finally it rebooted.
No app changes, using standard face, but did receive Samsung Pay upgrade this week. I am thinking it is related.
The same happened to me. I woke up today 4/05/18 to a black screen but it would still respond to phone calls and such. I would only feel the vibration though, the screen stayed black. I also held both buttons and it was back to normal after reboot. I also have the always on display activated.
Add another to the list of folks experiencing this issue. Has happened twice now, forced reboot both times by holding the power button down. I have a few custom watchfaces, but rarely use them and they have not been active when this occurs. Both of my occurrences have happened after the latest patch/update.
It happened with my gear too today and I tried the option of rebooting with both the keys pressed for more than 8 secs. And it worked. Guess Samsung should have some fix for this.