Why is my lg screen have no picture but has sound

Why is my lg screen have no picture but has sound

Little by little areas of screen are going dark and now when I turn tv on it has sound but no picture. To get it back to display picture I have to turn it on and off many times till finally the picture returns then sometimes it goes blank again, then I have to repeat the process. Pretty soon it's going to stay blank permanently. Could this be back lights, motherboard or something else? This is a lg model #50ga6400-ud 3d tv.

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