A few settings are worth checking before assuming a hardware fault. First, the AF-assist lamp does not light every time you half-press the shutter; it only comes on in low light, and some scene modes disable it entirely. So that alone does not mean autofocus is broken.

For the D3100, a common cause is the selected focus point. Try setting the autofocus point back to the center using the directional pad. If an off-center point is selected, the camera may struggle to find something to lock onto.

Also try resetting the camera’s shooting settings from the menu. A reset can clear an accidental setting change and has solved this issue for other D3100 users.

If autofocus still fails with both lenses after centering the focus point and resetting shooting options, then a service check may be needed.