Auto focus when I press the shutter half way down the camera will not focus. I can hear a "grinding sound"

I had the same problem with my 18-55mm nikkor lens I got it with my D40.
Somebody accidentally dropped my camera bag onto the floor from about two feet. the camera was put together inside and I when I put the whole thing in the bag like that, I usually try to switch the lens to manual so if it does bang around in the bag, the lens won't jam.
Unfortunately i forgot to do that, and the camera fell on its lens therefore causing a jam and causing the camera to squeak and grind and not focus whether it was in AF or M.
I was pretty bummed because that was my only lens, but when I looked up online how much it would cost to fix or replace, It was only about a hundred and thirty bucks to just buy a brand new one So I figured I might just see if I could fix this one myself. what could be the harm?
I switched the lens to M and carefully twisted the focus as far as the jammed lens would let me. Finally, I just had to be a bit tougher and twist harder, then there was a click, and it started to manual focus again. I don't think a broke any gears because I don't hear any rattling in the lens. If you think your lens is jammed like mine was, I suggest just trying to pop it back into place like I did, of course, your lens is probably WAY more expensive and you'd rather let a professional handle it.
But that's my story.
Hope I helped somebody!

Hannah

I would take the lens off and then put it back on, being careful to align the registration marks on the lens barrel and the camera.  It should click into place quite firmly.  If this does not fix the problem, take it back for repair.