What to do if your iphone screen goes black after dropping it

iPhone 4s dropped-now the screen is black

My daughter dropped her iPhone 4s (ios version 6) and now the screen is black (this was only four weeks after I replaced a cracked screen myself on this same unit and everything was working fine). She said it first landed from waist height on the top edge of the unit. Even though the screen is black, calls still come in (but I can't seem to answer them despite my swiping the screen area to answer), texts come in, the vibrate feature works, music was playing for a while and then I touched the screen where I thought the play/pause button would be and it the music stopped. When I hold the power button down to shut down the unit, I swipe where I think the "slide to power off" message on the upper screen area would be and in fact the unit powers off, so this tells me the digitizer is working at least in that area of the screen. I removed the back cover with the power on and when I hit the power button, I could see the edges of the back of the screen light up even though the screen was still black, so this tells me that some kind of power is making it to the screen. I then powered down the unit and re-seated all the connections to the logic board; it didn't help.

I read somewhere that restoring the unit to factory settings may work. She never backed up the iPhone to an iTunes account on our PC but did use iCloud backup. When I attempted to backup the iPhone to the PC iTunes account it required me to unlock the iPhone and every attempt I've made to "blindly" swipe and enter in the passcode has not worked. I didn't even get a vibration indicating the wrong passcode was entered.

If I restore the iPhone to factory settings, I read that it will update to ios 7. I've read and been told by many people that ios 7 is not optimized for the iPhone 4s and that it's a bad move to upgrade and best to stay with ios 6.

I'm hoping for some help here in (1) what may be causing the black screen and (2) will restoring the iPhone to factory settings work and if so, is there a way to restore the iPhone, given my situation, without upgrading to ios 7?