MacBook Pro display is dead but external monitor works – what gives?

My macbook pro (3 years old, the pre-unibody aluminum type)'s screen suddenly stopped working. I didn't drop it, nor is the backlight dead (I tried the flashlight on the screen test). There is no external signs of damage, and when I plug it in to an external monitor, I can see everything perfectly. It doesn't seem to be the logic board, since the external works fine. I was wondering what the problem might be?

Applecare ran out, and I'd very much rather try repairing this myself.

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