To test for this, plug your device into a charger or an iTunes enabled computer. Then shine a bright flashlight on the screen. If you can see a dim image, then the backlight circuit has been damaged. If you can’t see an image at all, then it may be the LCD circuit that is damaged.
In both cases, it’s either the actual LCD or the circuitry. Either way, those are both repairable but the board-level repairs are not really DIY unless you are already experienced in micro-soldering repairs.