Apple Watch flashing
Why does my Apple Watch flash like a strobe light for no apparent reason? This is happened to me twice in the last several days.
Apple Watch Series 6, watchOS 9
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Try a forced restart
You should force restart your Apple Watch as a last resort and only if it's not responding.
Don’t force restart your Apple Watch if you're updating watchOS. After you see the Apple logo and progress wheel, leave your watch alone and on its charger until the update finishes.
To force restart your Apple Watch, press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo