What Happened to my Nikon D3100?

I was at a park with friends and had my camera in my right arm and it got kicked out of my hand and fell on the rocks. At first I noticed a crack at the bottom but my camera worked ok. So I thought I just had to fix the screen. About an hour later, I Looked inside and it was really dark, everything is black. When I take a picture there's a black box covering most of it and you can only see the tiny top of the picture. So I know that it can take a picture, its just covered by a black box. I think it has to do with the reflector thats inside when you take your lens off. I'm really scared, and my mom is gonna get mad. Can I fix this by myself? If I can't, will it be covered by a guarantee? And if not, How much would the repair be?

Added (1). My LCD is not broken, Its the picture itself that turns out with the black box.

Warranty only covers defects in manufacture, not neglect. You can't fix it yourself. It takes a lot of skill to repair a camera.
There's a fee just to tell you what's wrong with it. And repair costs won't be under $100.
You'll just have to be honest & tell your mother what happened.

Well, plain and simple, you broke it. There's nothing you can do other than send it in. Now you know NEVER to hold your camera WITHOUT having the neck strap around your neck. There's a reason why it comes with the camera, and it should be obvious now why they do such a thing.

It will NOT be covered by the manufacturer's warranty. Unless you bought a warranty such as black tie from best buy, accidental damage will not be covered.

it will probably be $150+ to fix

You're screwed ***

Maybe you should be more careful with your camera. You have to tell your mom, and Haha good luck with that- you're dead