Phone dropped now it works iPhone 4

I dropped my iphone4 on the floor today, not a far fall about a foot or so. I stupidly left it on my gym bag and pulled my gym bag out from under it. Anyway when it dropped the screen just went black. It wouldnt pop back up or turn on at all. When I got home I plugged it in and it turned on and works fine. Can someone explain to me what happened? Thanks.
Gateway Windows Vista