Chosen Solution
I have spent some time on your site, and found similar problems I experience. My iMac screen is black to the naked eye, when plugging in a external monitor it works, and shining a flashligh on the screen reveals that my screen is still working, so my guess is the LED driver portion of the main board is faulty, is there anyway I can fix this?
It sounds like the backlight went out. I usually recommend screen replacement unless you have incredible micro soldering skills. You may have to purchase a damaged unit and use the screen off of it or purchase a new screen. Looks like a new screen is under $200 HERE And HERE is how to replace the screen
