Picture keeps get dark

@tvdoesntwork sounds like a bad backlight array. Start by verifying that you have no image on all inputs. If it remains dark, unplug it and push the power button for about one minute, then plug it back in. See if that resets your TV.

If it remains dark, use a flashlight and angle it against the screen with the TV at a station where you can hear the sound 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. then visually check the boards for any obvious damage like domed or leaking capacitors. Check all the wiring and unplug the wires and plug them back in to make sure they are properly seated. Post some images of all of your boards with your question. Use this guide iFixitでの質問に画像を追加する方法 for that so we can see what you see.