This is with the Sigma 24mm? If this is with the Nikon 24mm AF-D or another lens with an aperture ring then turn the aperture ring to its smallest setting (largest f/number) and control the aperture from the camera body the same way as with a lens without an aperture ring.
If it really is with the Sigma lens, try cleaning the contacts on the back of the lens and in the lens mount on the camera body.
Answers & Comments
This is with the Sigma 24mm? If this is with the Nikon 24mm AF-D or another lens with an aperture ring then turn the aperture ring to its smallest setting (largest f/number) and control the aperture from the camera body the same way as with a lens without an aperture ring.
If it really is with the Sigma lens, try cleaning the contacts on the back of the lens and in the lens mount on the camera body.