Hello, I'm TV Tech1 and I'll be working with you to identify and help you resolve your problem.
The very first thing we check - With the TV turned off and set in a dark/dim room (no direct sunlight) we turn on the TV and keep our eyes on the LCD screen. It should switch from a pitch black screen to a subtle gray light level indicating the backlighting has energized. Then we see if it stays on, or if it only lights up for a couple seconds, then goes out.
I've had to throw a dark blanket over me and the TV set in a brightly lit room to confirm the backlight did, or did not light up. You use what you have available to get the answer you need.