HOW TO recover from a completely darkened screen on Android device due to setting brightness to zero?

If the brightness control is set to zero on an Android 5.x device which has Adaptive brightness setting, it may result in display being turned off completely. Even if you restart the device, the boot-up animation will appear but the screen will go dark after booting up.

To recover from this issue, use another phone's flash light and direct it on the the front camera and light sensor. This will make the screen light up and you can proceed to change the brightness setting to normal. You can choose to turn off the Adaptive brightness setting y going to Settings > Display and disabling that option.

The Adaptive brightness setting is enabled by default in Android 5.x. It optimizes the brightness level for available light.