Why won't my Nikon D40 focus?

Whenever I try to take a picture, it won't focus and it makes this weird screeching sound. I've tried it in all different modes, and none of them will let me take the picture.
What do I do!

In your camera, it will only focus with a lens that has the focus motor in it. So it is likely the lens that is giving you problems.

Have you dropped or banged your lens into anything?

http://www.althephoto.com

Most likely the focusing mechanism in your lens. Two things to try:

1. Flip the switch on the lens to manual focus position and focus the lens manually. Then flip the switch back to auto focus. See if that helps.

2. Rotate the base of the lens back and forth without releasing the lens. This is an attempt to clean the lens contacts. Do not force this action. Or, you can remove the lens and reattach. The kit lenses I have come across on the D40 and D90 have electrical contact issues.