Nikon D3100 not focusing automatically?

So I rarely use the automatic mode in my DSLR, but one day I did.everything was on automatic mode including the lens but I noticed that its not focusing automatically. No sound or adjusting.it'll just snap even though it blurred. You guys know the reason why this happen?

Added (1). The lens is set at AUTOMATIC mode

Switch the switch on the side of your camera body to AF(auto focus). Not MF(manual focus).

Maybe your lens switch is set to mf? For manual focus

The location of the switch could be the problem, the lens is not an AF-S lens could be the problem, or the camera could be set (in the menu system) to release priority instead of focus priority. That means the camera will shoot when you press the shutter button, whether or not the camera thinks the photo is in focus or not. Focus priority means the camera will not open the shutter unless it thinks the image is focused.

Check the menus.

Two settings can affect focus mode.
1. Be sure the switch on your lens is set to auto focus mode (depending on lens, auto setting is marked A or M/A). See page 16 in your manual. Since you state this is set to auto mode already, see the second item…

2. Be sure the menu settings are set to some sort of auto focus mode (AF-S, AF-A, AF-C).
See page 55 in your manual. Page 38 has info if you're using live view.

Side note, the D3100 will only auto focus with an AF-S style lens.AF style lens won't auto focus on this camera.

AUTO should bring everything auto except the lens where it has AUTO hard switch. But since you have it switched to A, then no reason. Except a software bug on the camera.

Reset it and try again.

The D3100 does not have an AF motor so I guess it's the lens that snapped. If you have set to shoot with AF on and you turned the focus ring of the lens manually, that could be one cause. Setting the camera's mode dial to Auto is not related to autofocus at all.

To check, try focusing on a brightly colored parked car under the sun. Stay more than 10 feet away from the car. Set the camera to Auto everything (mode, ISO, WB) with EV set to 0 (zero). If the camera can't focus and take a decent shot then bring the whole thing (camera and lens) to the authorized service center. Ask the store you bought the camera from where to find it.