Q: Macbook Air display dead, everything else works.

My mom gave me her disfunctional 2014 MacBook Air after she purchased a new one, and I'm trying to diagnose the problem with it.

Currently, the computer powers up, but the screen doesn't work at all, its completely black. I plugged in an external monitor, and with the mirror-ing key command i can get the machine to work completely on the external monitor. I tried all of the fixes for the 'black screen' software errors people have been reporting and none of those work. My mom said she was wiping down the computer with Windex and may have sprayed too much on the screen/keys. Low and behold  it didn't work after that. I'm not paying to get this computer fixed at the Apple Store, but I want to try and do a replacement myself. Does this sound like a screen replacement or a logic board replacement. Due to the fact that everything else works, I'm a little unsure.