screen goes dark when movie is on sound still works

@karenseiler 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. Now when you have sound but no picture, use a flashlight and angle the light at the screen on different areas. You might make out some shadows or see a slight grey color. IF you do it is the backlight that has failed. It is always possible that this is related to the backlight circuit on the power supply board. It is also possible that this is a T-con board issue, but in that case you will not see any shapes etc.

To verify this we would need to see the boards etc. Remove the back panel and take a picture of all the boards. Post those with your Question. That way we can see what you see. You will need a multimeter for this . It is not very difficult and is actually pretty straight forward. If you are not afraid of using some simple hand tools like a screwdriver, it can be done. :-)

Adding images to an existing question