@kathy1964 I recognize this is an old question but it does not appear to have any valid answers. Since “If you shine a flashlight on the screen you can see the picture. “ you already determined that this is a backlight issue. Your TV does not have a separate backlight inverter etc. Resetting it, or turning ift off and on will not fix the backlight.
You will have to measure the output of the power board to see LED backlight array. You can measure that directly on the connector at the power board. Samsung TV’s have the values expected, printed on the circuit board. Measure those and make sure that they are all in compliance. If you have less power as expected or none going to the backlight array, you have a power board issue. IF you do not get teh BL_on signal from the main board then that is the issue.
To test the actual backlight array, all you have to do is to remove the back of the TV and disconnect the cable from the power board to the main board. Then turn your TV on. If you do not get teh backlight turned on, it is the backlight array that has failed.