Why my Screen is dark?

I bought my MacBook white unibody may be 5 years ago.its been 1 year I am not using it. The problem was my screen went dark.i took it to Apple Store and I had to replace the mother board. I didn't do any thing. My laptop still looks like working as it plays song when I randomly press buttons. But the screen is dark.hdmi does not work either.

Do I really need to change the mother board to fix it? If so does it worth to? How much?