screen diming

I have a toshiba lap top thats about ten years old and for a while now it has been going black screen or so i thought. The screen is actually going really dim, i thought the computer was shutting down but it is not it still running. I have hook up a new monitor and it runs perfect. I have found out that i can play with the little button that shut the screen down when you close the top (screen) down and the screen will get brite and stay lit until i shut it back down. Heat or time it is run dosn't seem to matter, i am lost for a solution.