Screen not turning on.

Hello, I was fixing a friends iPhone headphone jack and that went successfully. The phone turned on and worked fine. As I was closing up, my elbow hit the iPhone and it fell about 4 feet to the carpet. There is no visible damage. I tried to turn it on, but all that I saw was the back light. I tried to hard reset, (hold both buttons for 7 seconds) and it turned off, but now the iPhone isn't showing any signs of life. The computer responds when I plug it in but that's about it. I have checked the connections many times and all the connections seem fine. Any tips on what to do?