Why is my iphone screen black?

I got my battery replaced a couple days ago. I realized that a screw (on the bottom near where you connect the charger) is missing. I didn't pay too much attention to it and thought it wasn't important. Yesterday in class my phone fell out of my pocket. Keep in mine that I have a thick battery case AND my phone fell on it's back. Nothing is cracked. The phone still works I can hear when someone calls, texts, and I can use voice control it's just my screen is BLACK. What wrong with it? What can I do and how much will it cost to repair it? I don't have apple warranty.

I had same problem i took my iphone to these guys they fixed it celphone@optonline.net