More than likely you have 1 or more LED's that are shorted out. When this happens the LED strip will draw too many amps and the board will shut down all LED lights as an over-amp precaution. This happened to my 50" LG and I was able to fix it at no cost by removing that particular LED strip from the circuit. It is not a difficult repair, but does require someone to pretty much completely disassemble the TV. If you want to attempt the repair, here is a Youtube video that provides some instructions,
You can replace the strip that is bad for about $50 or just remove it from the circuit. If you just have one bad strip, you probably won't notice any change in picture quality after it is removed. Be careful if you disassemble the TV. Even if it is unplugged it will still have capacitors that carry a high charge of voltage.