What is the red lightning bolt on my Apple Watch?

Apple Watch not responsive? What does the red lightning bolt mean? Apple Watch is only showing the time, how can I exit Power Reserve mode?

Apple Watch power reserve mode

When the Apple Watch displays a red lightning bolt, battery life is critically low. The watch has automatically entered Power Reserve mode, and the device can only show the time.

The only way to remove the red lightning bolt and leave Power Reserve mode is to charge the Apple Watch. Placing Apple Watch on a charger will restore battery power and resume normal operation.

When the Apple Watch is placed in Power Reserve mode with a charged battery, the red lightning bolt does not appear. To exit Power Reserve mode with a charged battery reboot the Apple Watch.