Ummm. Just did a bit of digging. Apparently, it's the Apple watch BSOD (black screen of death) I did a force restart and voila! back up and running with 75% baddery!
Me thinks I'll wipe the watch and start again - maybe after tomorrow with the new watch IOS coming out.
That doesn’t sound good in my opinion. It may well be a hardware fault that won’t get resolved. It does have a standard one year warranty so personally I would just make an appointment at the Genius Bar and get it swapped out. If you have Apple Care Plus all the better.