Backlight on laptop comes on but nothing shows on screen.?

One day I was on my laptop and the screen faded out and went to black. Since then, I have not been able to get any image on my laptop screen. The screen stays black. I can plug my laptop into an external monitor and I am still able to use the laptop. The image shows up fine on the external monitor if I hit the CRT/LCD on. Thinking that the screen was bad, I replaced the screen with a brand new LCD screen and I get the same results. When I turn on the laptop, I can tell that the backlight comes on because the screen gets a little brighter, but still no image shows on the screen. For reference, my computer is a Dell XPS M1530 laptop.