I did that and it’s still not shutting down.
I tried force shutting down my watch and it is still not responding.
Apple Watch Series 3, watchOS 7
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That is power reserve mode,let the watch charge for an hour or so then;
To turn off power reserve mode,press and hold the side button for about 10 seconds, until you see the watch restart and the Apple logo appear.
Keep in mind that if your battery is almost completely empty, you might need to charge the watch before it will restart normally.