I don't know what happened with my Sigma lens. Basically when I mount it on my camera everything is dark and auto focus cannot focus. It's like not enough light is coming though.
When I put other lenses on, everything is OK.
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It sounds to me like the aperture control in the lens has failed and it will need to be serviced.
All Auto Focus camera lenses are wide open until you actually take the photo. Look though the front or rear of the lens, and it should be completely open. If you can see the hexagon type shape of the aperture blades, it means they are not working properly. You will not be able to auto focus and the viewfinder will be very dark.