That message usually means the camera and lens are not communicating electronically.

Since your other lenses work on the D3100, the body is less likely to be the problem. Likely causes are:

• poor or intermittent contact at the lens mount contacts
• a lens fault, such as an internal ribbon cable or connection coming loose
• a compatibility issue if the camera firmware was updated and the older Sigma lens no longer communicates properly

Good next steps:

1. Test the Sigma lens on another compatible Nikon body if possible. If it fails there too, the lens is the likely problem.
2. If it works on another body, check whether your D3100 recently had a firmware update.
3. Contact Sigma support/service. Some third-party lenses need a firmware update or rechip to work with newer camera firmware, and older lenses may need factory service rather than user updates.

Because you’ve already cleaned the contacts and your other lenses work, this is most likely either a fault inside the Sigma lens or a Sigma/Nikon compatibility issue rather than a general camera failure.