Why does a lap top screen get dark?

LCD is run by the video card. Also, it will be dimmer if running on battery versus off of your adapter...energy saving technique. One thing you can try is find the Fn key, and then another key with a sun on it. The ones with the sun is on my arrow keys since that is the button you push, along with the Fn key, to make your screen brighter or darker. Someone may have pressed those keys without knowing it. If those things do not make a difference then you will probably need to be getting a new video card soon.