It took me at least a year to find this...
Change the LCD brightness
From the Windows desktop, hold the Fn
key and press the F7
keys to increase or decrease the brightness of the display. If using the function keys does not resolve the problem, determine whether the Lid Actuator switch is stuck.
The key number may vary depending on the notebook PC. The image on the key should resemble the following images: Figure 1: Brightness Level Down
Figure 2: Brightness Level Up
During battery power operation, the notebook display will dim to reserve battery power.