Where can i go to fix my d3100 lens - Nikon D3100 Digital Camera

Look here for specs: Nikon D3100

make sure the lens selector is set on automatic not manual.
check that link, maybe something there might help?

You need to try it with different lenses, If it does it with both then its the body, If not then its the lens, Usualy if its the body, Then it has to go to a Camera repair shop like me because the fix is on the side of the mirror box,

Yes, as long sa the lens can fit the bayonet. But it can't focus, because there are no AF motors in Nikon D3100 and AF 70-300mm f / 4-5.6 - G. And Nikon lens with AF is without AF motor. Lens marked with AF-S have focusing motor

The lens is a manual focus lens. It doesn't matter which mode you have the camera in, you'll still have to focus it yourself.

The D3100 doesn't have digital zoom capabilities. The zoom range depends on the lens you have fitted on. The 5.6 sounds like the aperture. The maximum aperture also depends on the lens.

The S3100 does have digital zoom capabilities. Is this the camera you're referring to? Or are you confusing the specs of the two cameras?

You didn't specify the make of the lens. Assuming the lens has a Nikon mount then yes, it will mount on a D3100. The big question is whether it will autofocus on the D3100. The D3100 does not have an autofocus motor, relying on the lens to have one. Not all lenses do. Nikon designates lenses with an autofocus motor as AF-S. Other manufacturers use different designations.
If you need further assistance, feel free to reply to this post and give the make of the lens.

No. This lens does not have an internal motor which the D3100 requires for autofocusing.

The D3100 can't autofocus with this lens. The D3100 does not have a focus motor, depending on the lens to have one. The AF-Nikkor 50mm doesn't have a focus motor, depending on the camera to have one.

To autofocus, you need lenses designated AF-S.

If those lenses have a Nikon mount then they will fit on the D3000 and D3100. However, the D3000 and D3100 lack a focus motor. If the lens does not have the focus motor then you will not have the autofocus capability. Sigma calls their focus motor the HSM; if the lens does not specify HSM then they do not have the motor.