This is most likely a compatibility issue rather than a camera fault. Older Vivitar lenses often lack the electronic contacts your Nikon D3100 expects, so the camera may report “Lens not attached.”

You may still be able to use the lens, but typically only in fully manual operation:

• Set the camera to Manual (M) mode.
• Set shutter speed on the camera.
• Set aperture on the lens.
• Expect no autofocus and usually no in-camera metering.
• Check exposure using the rear screen/histogram or an external meter.

Also check the physical connection:

• Clean the lens and camera contacts carefully.
• Make sure any spring-loaded contacts are not stuck.

If the lens mount itself is not the correct Nikon F mount, or uses an incompatible fixed mount, it may not function properly on the D3100 at all. In short: the message usually means the body cannot communicate with that lens, but a manual-exposure workflow may still work if the mount is mechanically compatible.