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