There are three possible reasons that would cause you to face this issue;
* Power board is faulty and/or dead. This would require a replacement of the board
* Main board is faulty and/or dead. This would also require a replacement of the board.
* Backlight LED Strips are fault and/or dead. This would require a complete disassembly of the T.V. to expose the LED Strips in order to manually test each LED to ensure they are working.
The issue is that it could be either one of these three issues or it can be a series of them. I would recommend that you start with the power board then the main board ending with the backlight as the process involved to repair the backlight LED strips requires advanced technical knowledge and time.