DIAGNOSIS
The display has timed out or entered sleep mode, but the device itself is still powered on and functional.
FIX
Wake the display. Tap the screen firmly or wave your hand in front of the camera. If that fails, press the Volume Up or Volume Down button on the top. The screen should illuminate. Adjust the screen timeout in Settings > Display > Screen Timeout.
IF THAT DIDN'T WORK
Check the ambient light sensor. Ensure the device is not in direct sunlight or extreme darkness, which can confuse the auto-brightness. Cover the front-facing camera completely with your hand for 5 seconds, then uncover it to trigger a sensor reset.
HEADS UP
A completely black screen with no backlight at all, even when powered, indicates a potential hardware failure. Before assuming the worst, try a hard reset by holding the Action button for 25 seconds.