Stuck on boot loop.
The watch wasn’t dropped or submerged or anything radical like that. One day the watch was working fine, the next day it wasn’t.
At the start, the watch wouldn’t turn on if it wasn’t in the charger. (Tried OEM Apple charger and a third-party charger, same results.) When on the charger (either of them), the Apple logo appeared for about a quarter to half a second, then the screen went black for four seconds, then repeat forever.
Still on the charger, I held down the side and crown buttons for about 30 seconds and got the red exclamation mark in the circle and a URL to www.apple.com/help/watch. Following the instructions there I held down the side and crown buttons for about 10 seconds, and when the Apple logo appeared I released both buttons. The Apple logo stayed on screen for 12 seconds, then the screen went black for four seconds, then repeat forever.
Even after being on the charger for about half an hour, the watch will not turn on if I remove it from the charger. If the Apple logo is on screen and I remove it from the charger it immediately shuts off.
It consistently draws about .2 amps, slightly less (about .18 amps) when the Apple logo is NOT on screen. The watch is hot but not burning; if it was on my wrist it would be uncomfortable but not painful.
EDIT 1: After posting this I unplugged the charger to move it to a different table. Upon plugging it back in, the watch went back to its “fast boot loop” cycle (quarter to half second of Apple logo followed by four seconds of nothing). Holding the side and crown buttons does nothing; the watch is currently in this “fast boot loop” cycle.
EDIT 2: About 10-15 minutes or so after the previous update I went to check on the watch. It was no longer in its fast boot loop; the screen was dark and it was not drawing any current at all (0 amps). Holding the side and crown buttons for more than a minute, or just the side button for more than a minute, did nothing. Removing the watch from the charger and repeating the button holding exercise (for both side+crown and side only) did nothing. Putting the watch back in the charger did nothing. However, when I unplugged the charger and then plugged it back in, the watch came on and started its fast boot loop process again (drawing about .2 amps all the time).