Use this guide to force restart your Samsung Galaxy Watch.
Push and hold the lower button for at least 7 seconds.
Let go of the button when you see the watch restart.
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I suppose all commenters above have tried to charge the watch over night to make sure the battery is not completely depleted.
You could try and restart WHILE it is charging or even just placed on the charger without being plugged into the wall.
I dropped my watch off the counter and it's just keeps showing rebooting it won't let me restart watch or activate it from my phone.