If it has sound but no picture then one of the following parts is faulty, and must be tested and replaced:
The inverter is the high voltage board that provides variable power to the backlight lamp. This is the most common. When inverter is faulty you can get no picture, or a very dim picture that can only be seen by pointing a torch light with a 45 degree angle on the LCD surface from very near distance.
The inverter must be tested by reading voltages with a multimeter.
The LCD ribbon cable/LCD display and connectors.
This is quite uncommon, but happens. If the TV had been hit or moved the LCD cable may have slipped out, or the LCD connector may be damaged.
The LCD controller board.
This is the board that controls the video signal, it is difficult to test, as there is need to read determined values on IC pins. To test this board you need to purchase a service manual and follow the troubleshooting diagrams on service manual.
From description it looks like a faulty inverter, I think that is the first part to test.
If under warranty contact the manufacturer ringing the number on the user manual.
If you want to repair at your own expenses, just locate any good qualified repairer.
If you want to DIY you find parts here,
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