This solution covers and may possibly help fix a not working LCD display backlight LED and shows a dark screen display on the device.
If this problem occurred, you may first try to replace or install new LCD screen module just to easily determine if the LCD backlight LED is already damaged or caused the problem.
If after changing the LCD screen and the problem still exist, check and try to replace the flex ribbon cable wire. If the flex seem working, Reheat or then replace the LED driver IC, but check the 3.7volts supply voltage to it first before attempting to replace it. You can check this voltage on the filter coil showed on the solution above.
