Turns on but there's no picture

Goin by the only info youve given it sounds like either youve got it on the wrong signal source, backlights have went out or the power board for some reason is t sending power to the lights!

@adahart you have not told us what you have checked. The no video issue can be related to a bad backlight inverter circuitry or a bad backlight array. A simple check for that would be to darken the room and turn the TV on. If it remains dark, turn the TV to a channel where you can hear the sound. Next, use a flashlight and shine the beam at an angle against the screen. See if you can make out shadows or shapes. If you do, then it is your backlight that has failed.

If you do not see anything, unplug your set and remove the back cover. Then visually check the boards for any obvious damaged components. Check all the wiring and unplug the wires and plug them back in, make sure they are properly seated. Post some images of all of your boards with your question. Use this guide Adding images to an existing question for that so we can see what you see. It is possible that this is either a power board or a T-con board issue.