Why did the screen go blank but sound is perfect

@musbred, check all the video inputs first. Try each one separate. disconnect your TV from the power and disconnect all input sources. then hold the power button down for ~30-60 sec. then reconnect to power and connect one input source etc. Let us know if that made any change.

If it did not then in a darkened room shine a flashlight beam against the screen (at a channel where you have proper sound) see if you can make out any shapes or silhouettes.

The remove the back from your TV and disconnect the power board from the main board. See if that turns your backlight on. Check all the boards for obvious damage like domed or leaky capacitors. Post some images of all of your boards with your question so that was can see what you see. Use this guide Ajout d'images à une question existante for that,