My LG LED TV has a black screen. I am unsure if it has sound due to a sound bar being hooked up to it prior to it not working. any advice? what parts do I order?

I plugged my cable box into the TV to attempt to get sound and came up empty. I am assuming that could be because of various reasons such as: One or more of the boards is shot, because of the sound being set to a sound bar, or because I couldn't see what I was doing and was not on the right input. Another thing I looked into was the back lighting. This is problem that I haven't seen anyone else have. There is, but there isn't back lighting. let me explain.. when you turn on and turn off the TV, the screen flashes very briefly. I tore the back off of the TV and in a dark room, turned the TV on and off again. I could see that all of the lights flashed at once. They are all working but do not stay on. I would compare it to a camera flash. This leads me to believe that is something to do with the power supply/LED board. Like I said before, I haven't been able to find any info pertaining to my problem. I have found info and videos of the LED's flickering, which is a problem with the LED strips. But my TV is not flickering. I just want some advice on how to fix it and what parts I need to order. Thanks in advance! Serial # 50LN5400-UA