Try the following:
Turn off the TV if on and then disconnect the power from the TV.
Remove the back cover from the TV and then disconnect the cable between the power board and the mainboard, at the power board end.
Reconnect the power to the TV.
The TV backlights should turn on full intensity and stay on. There will be no images or audio because the mainboard is disconnected.
If the backlights blink or don’t turn on , then there is a problem with either the power board or the backlights.
Use a DMM to measure the LED voltage on the power board LED power connector between the LED+ve pins and an earth point (screw head of screw holding board to chassis). If you measure a steady >220V DC then there is a problem with the LED backlights or the connection to them. If you measure <70V DC then there is a problem on the power board.
If the backlights turn on and stay on, measure all the voltages on the power board’s mainboard cable connector (where the cable has been temporarily disconnected from to do this test) and check that they correspond to the voltages that are usually printed on the board close to the connector. If they are all OK it may be a mainboard problem.
Be safety aware when testing as there is exposed lethal AC voltage on the power board near to where you will be testing. If you don’t know how to test safely when AC power is present then don’t do it! Contact a reputable, professional TV repair service and ask for a quote.