Most probably this is not a fault but a BIOS setting, caused by a settings reset or a discharged buffer battery.

To reset the backlight, simply switch on the product by repeatedly pressing the delete key to access the BIOS, once inside navigate to the advanced > LVDS Configuration tab and set the following items as indicated:

Inverter Polarity [Inverted]
Mode Select [8-Bit VESA]

At this point, saving the configuration and resetting the machine should bring the brightness level and functionality back to normal.

If it becomes necessary to perform this operation every time the ignition is switched on or at any rate frequently, this will almost certainly indicate that the buffer battery is flat and it needs to be replaced.