That was kind of a common problem with cheaper laptops a while ago, The display is connected to the mainboard of the notebook with a flimsy ribbon cable and the backlight most of the time by a seperate power cable. The problem was most of the time that one of the cables came lose and then the lose connection only closes in a certain angle (most of the time by opening and closing again in a few times) The solution was to open the notebook and firmly plug the cables in and secure them with tape or something similar.