Laptop display not working

I have A Dell Inspiron 1100 with a very perplexing problem. The display does not work in Windows. I can see the post, I can see setup, but once the windows logo shows it goes blank. I can plug a monitor in and it works perfectly. I see everything including Windows. I press the Fn + F8 to switch between LCD & External and nothing happens. The display stays on external. Any ideas?