turns on for about 20 seconds then goes black but can still hear it

@ljjsmom3 let us know what exact model your TV is. Once it has a black screen but maintains the sound, darken the room and shine a flashlight at an angle against the screen. If you can make out shapes/silhouettes etc. then you know that this is a backlight issue (bad backlight array or bad power supply). If it does not show anything, it is most likely a main or t-con board situation.  The backlight is inside the display panel. It can be done but for a panel the size of your TV you want a second set of hands to help you out. That way the chance of flexing and breaking your panel is less.