Chosen Solution

What’s is wrong with my lg 75UJ6450 the screen black but u can hear the sound

Typically when I see an issue like this, it is backlight failure. Could be the inverter if all the LEDs went out at once. Try shining a flashlight into the panel, will probably have to be pretty close if not touching the TV. If you see an image, then it is likely backlight related. Further diagnosis will require teardown of the TV at that point.