Shoutbox

My first thought was that you may have accidentally switched to external display only mode -- another of the brand-dependent key combinations, usually Fn + some function key.  You should try to find this key combination for your laptop (the function key involved is typically marked with a symbol that looks like two screens) and press it a few times, just in case.

However, my second thought was that's probably not it: when the built-in screen is switched off, it is off -- there is no display on it.  You had display, just no backlight.

You should rule out the external display switch and also reboot the laptop and observe, in case the screen lights up initially and only goes dark after Windows boots, which would point to some software problem. 

Lacking either of those, it'd seem that you have a backlight or backlight inverter failure, which would have to be repaired at a shop.  I hope the laptop is still under warranty -- otherwise, you may be looking at a pretty expensive repair.