Apple watch
My watch was working fine yesterday now it’s only showing a red lighting bolt and the time
Apple Watch Series 3, watchOS 7
User profile for user:
Community User
It's in power reserve mode,fully charge the battery and;
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.