If an external monitor is working that nails it down to the internal display and its connections and the backlight logic.
With everything connected using a flashlight pressed on the screen at a sharp angle are you able to see the faint image of your desktop and the icons on it? If you are your backlight cables & logic needs to be checked out. If you don't see anything in a darker room restart the system to you see a quick faint glow during the boot up (sometimes you might not see it)? If you do then the sync cable is bad.
Otherwise the display its self likely needs to be replaced. Did you bang or drop the system?