There are two common problems that would make your screen no longer work: the screen is broken or a bad logic board. If your having more than just screen problems it's most likely the logic board is bad; the way to fix this is to replace the logic board. If it's only the display that's not working your screen is probably broken. The way to fix the broken display would be the replace the screen.
There are a wide variety of problems this could be. There are some different things that can be done to fix this. Two of the main problems are bad wiring or bad speakers. The most common way to fix this problem is to replace the speaker.