It sounds like one of two things has happened to your device:
* If you can still see images on the screen, but it is very very dark, to the point of being unusable without squinting, your backlight has probably cracked.
* If you cannot see any images on the screen, but you can see a difference between On and Off in how bright the black screen is, then your backlight is fine, but your LCD is likely either not fully connected, or otherwise cracked/separated.
I suspect it's the latter, due to the reference to the screen turning yellow. If you remove the front glass/digitizer, you can check the connection to the board. If reseating the connection fails to solve the problem, I'd recommend replacing the LCD, as that sounds to me the most likely point of failure in this particular situation.