My nikon D50 won't focus?

It will take very blurry pictures. I already popped out the battery and put it back in…

Do you have both the lens and body set to auto-focus?

Why do you think that "popping" out the battery will do anything on a dSLR

Blurry photos are the results of two things when shooting at slow shutter speeds.

* movement of the camera during the exposure
* movement of the subject during exposure.

Take your cameras outdoors in full sun and take some shot with your shutter speed set to 1/250th second or faster

You really need to spend some quality time with your user manual

This link will help you understand how to balance ISO, shutter speed and lens aperture.

As mentioned, make sure that your lens is set to shoot in the auto-focus mode

Edit: I hope you know the difference between an image being blurry and being out of focus. They are two totally different things, caused by totally different things

The D50 has an AF/M switch on the body, not just the lens. That switch is for the older type AF-D lenses that link to the camera body's autofocus motor. Although the AF-S lenses don't rely on the D50 body's motor, switching the body to M cuts off the electronic autofocus interaction.

http://keerok-photography.blogspot.com/2011/12/blurred-shots.html

If it's not one of those, your camera or lens might be broken.