First, double-check the AF/M switch on the side of the 35mm lens itself. It’s easy to bump it into manual focus.

If the switch is set correctly, listen for the lens when you half-press the shutter:

• If you hear no focus motor activity at all, the lens likely has an internal fault and will probably need service.
• If you do hear the motor but the lens still won’t focus, the problem may be in the lens’s internal focus mechanism, which still means service is likely.

Since your other lenses autofocus normally on the D3100, the camera body is probably fine and the issue is isolated to the 35mm lens.

You can also try removing and reattaching the lens and carefully cleaning the electronic contacts again, since a poor connection can sometimes cause AF failure. If that doesn’t help, an authorized service center is the practical next step.