It happens usually after you accidentally press a wrong key or you finish updating your operating system. If you are a Lenovo laptop user, you may have encountered a very annoying issue that the screen of your laptop went very dim. Some ThinkPad models dim the display automatically when you unplug them from an AC power source. Press the 'End' key again to dim the screen more.
Hold down 'Fn' and press the 'End' key to decrease the brightness of the display.
