My LG 50" TV has sound but the picture is black

My LG 50 inch flat screen TV has a black screen but, it still has sound. You can turn the room lights off and put a flash light to the screen and see a picture. A so called TV repair man told me to get a power board/control board kit, after charging me a hefty house call fee. I installed all of the boards that came with the kit. My screen is still black and it still has sound. Someone told me the LED back lights might have burned out. How do you fix the LED back lights on a LG tv, model #50LN5100-UB and do you sell the parts the repair manual or know where I can get the parts and manual?