Why is my screen black, but still responsive?

A friend of mine dropped my phone (which happened many times before and to worse degrees), however this time when I reassembled it, the screen did not come back to normal. The buttons turned on, the screen lit up black with colored stripes. I can still get calls, receive alarms and hear music, all based on my memory because I can't see a thing on the screen. Does anyone know why this happened, and how I can fix it? I tried hard reset, but that only made half the screen light up white, and the other half stayed all striped. I have done this many times and concluded that the whole screen works, but for some reason no images show up.