MacBook Pro display black, external display works

Hey,

My wife's early 2011 MBP has a problem. It is connected to an external monitor, which works fine (using it to post this), but the built-in laptop display is black. I have done all the restart stuff to reset PRAM, SMC, disconnected everything, &etc. There is absolutely nothing showing up on the built-in display, no backlight or logo.

I have not opened the case yet, since I have a feeling this is some sort of software issue. Can anyone think of any other non-invasive fixes, or should I go ahead and open the case to check the video cable?

My wife just mentioned that this began after the computer had been unplugged and run its battery down until it shut down.