Nikon D40 will not autofucus.

You cannot take a picture when your camera has not been focused yet. This is done by pointing your selected focus point on your subject, then click the shutter button halfway / half-pressed until you here a beep or see a dot on the lower left portion of your viewfinder. Once you have locked your focus, press the shutter button all the way down to take the shot.

Does this problem still exist when you have the shutter pressed halfway down?

Regards,
Worldvet

You can change this in the menu while in shooting mode. This is from page 48 of the manual:

Choose how the camera focuses.

A (auto) M d (Shooting menu) M I Autofocus mode

A Single AF (default setting)

The camera focuses when the shutter-release button is pressed halfway.

B Full-time AF

The camera focuses continuously until the shutter-release button is pressed down

halfway. Use with moving subjects. The sound of lens drive movement will be heard

while the camera focuses.

If you can dare servicing youself You can open bottom cover of and turn a little red wheel manually. try pressing the release button now and will start working. If you have this problem re-accuring, then get it checked wih professional.

Any of the autofocus settings will attempt to focus when the shutter is pressed halfway. What lens are you using? If it doesn't have an internal focus meter, then the D40 won't autofocus with it no matter what. If it does have an internal motor, then check to make sure the lens is set on autofocus.

Check Custom Setting 12. If it's set on AF-On then the shutter button won't focus.

If this doesn't fix it, specify the lens and any other details.

The D40x only autofocuses with the latest AF-S (and old professional AF-I) lenses. Most traditional (screw-type) AF lenses from 1986 through today will not autofocus on the D40x. If your lens says "AF-S," you're fine.
Most lenses sold today for the D40x are AF-S, and so is the included 18-55mm kit lens. Many non-Nikon lenses from Tamron, Tokina, Quantaray, Sigma and anyone other than Nikon still won't autofocus on the D40x. Try before you buy, but honestly, there isn't much the included 18-55mm lens won't do.
Autofocus systems are fast, but not usually instantaneous. You have to hold the shutter down halfway as you compose the first shot so the AF system can focus and lock, and then the camera fires instantaneously when you press the shutter the rest of the way. See Preventing Shutter Delay for more.

Look on the left side of the lens. Make sure the button is on A for auto focus and not M for manual. OR not stuck in the middle. Hopefully that will solve the problem.

Camera wont let you take a picture until focus is locked, why are you using M (manual focus) if you expect autofocus to do it for you? In M mode it is up to you to focus correctly or camera will not take the picture.

When the camera is in auto focus mode, it will not take the picture until it locks onto a subject. If there is not enough light, the camera will "hunt" for something to focus on. Since it can't define any edges, it won't allow the shutter to release.

The D40 base model does NOT have a powered focus lever for autofocus, so if the autofocus is not working, suspect the lens.

This happened to me once on a D70... When mounting the lens, make sure it clicks in all the way when turning counterclockwise. Also it may be worthy to note that both the lens AND the camera must be set to autofocus mode ("A" switch set on both lens and cam).

Marky