How to fix that backlight problem?

@jeonseol

there are a few things that could be causing this. First check the wiring going from the computer to the screen. Check the area around the hinges. It is possible that the wires got damaged/cut. Then it is possible that the screen itself has failed (backlight array). Lastly it could be the backlight circuitry on the motherboard. For that we would need an exact number off the board so we could check and see if any schematics are available. That would show the circuitry and location of components etc. When you get to that point, post some good pictures of your motherboard from both sides so we can see what it all looks like. Use this guide to post your pictures with your question Adding images to an existing question

you can download the mainenance manual from places like this