My dslr camera focus doesn't work?

I bought my dslr camera, specifically a nikon d3100. The focus doesn't work anymore for some reason. I bought it from bestbuy. What should I do with it? Can bestbuy give a new one?

Added (1). It fell a couple of days back.

You need to read your user manual and then learn how to use your cameras controls

You made some change in the camera/lens configuration, maybe without knowing it.

Look on the lens for a switch, MA/M Make sure it is in the MA position

You've probably answered your own question with the words "it fell a couple of days back".

The D3100 does not have an autofocus motor built in. So there are two possibilities:
1) You switched the camera from AF to MF. Refer to your user manual as to how to change that back.
2) The lens is the problem. It is either broken, or the A/M switch is set to M.

To test the lens, stick it onto another camera (find a friend with a Nikon), and see if it will focus.

Check your lens barrel make sure its in AF not MF, if still not working get the camera back to the place you bought