Follow the excellent troubleshooting instructions in the YouTube video:LG No Picture, Screen Flash 47LN5200 47LN5400 47LA6200 47LN5700 47LN5750 47LN5790

Take care to unplug the LED power connecter before you test with the power supply!

Once you have determined where the failed LEDs are, you have a choice:

Purchase the replacement LED strips needed; be sure to order the correct part number. Note there are several part numbers for the various strips.

Replace all of the LED strips.

Replace the individual LEDs, using the heat gun method shown in the video. This isn't as hard as it looks. But you'll need a steady hand and good vision to properly place the new LED(s).

Be sure to remove the two-sided tape from the LED strip before heating!

And, replace the removed tape with new double-sided tape.