Are your shutter speeds normal? What you are describing sounds like a stuck aperture - something that would require exchange or repair. Change to Aperture priority mode and see if you can adjust the aperture. Try a near wide open aperture - like f/5.
I had this happen once with a new lens - very frustrating. It's a minor issue but probably more easily resolved by exchanging the camera and lens for a replacement kit. In my case I sent the lens for warranty service since it had a 3 month wait for a replacement.