My screen is black but there is sound

Crystal Smith verify your model being a LG 60LM6450. You do need to check all inputs and see if the screen remains black. Get a flashlight and in a darkened room shine the light at an angle against the screen, With the TV at a station that you know is functioning, something where you have audio, try and check to see if you can make out silhouettes. If so, you know that your backlight is not working. You can also remove the back of your TV and see if you can make out the backlight coming on by checking the back of the panel. If not, then your issue is most likely the power board. It will help if you 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. It will allow us to see what you see.