Pierre is right-I'd look for trouble within the camera, not the lens. Except...
Take a good look at the electrical contacts between lens and body, check for cleanliness and good contact. Also, most important, check the small cam that operates the diaphragm (aperture) for proper operation. With lens in hand, stop it fully down. You should see the diaphragm blades leaving only a tiny non-round opening. Now, gently pull the small lever you'll see in the lens mount (in the black part, as you look at it from camera body position, you'll see it at around 3 or 4 o'clock, so to speak). As you move it, you should feel spring resistance and see the aperture increase. If it feels dead the spring probably broke.
Lastly, check the following with the (filmless)camera: with no lens attached, fire some shots and see that the cam actuator moves. This is a small lever you'll see on the left of the mirror box, just by the lens mount. It couples with the lever you checked on the lens, and it should move up at the beginning of exposure and down at the end. Hands off the mirror box while doing this, try not to touch anything in there. Now take your finger off the shutter release and GENTLY try to move this lever up. It should show resistance and not move, if it does, it might be broken.
If none of these help pinpoint the problem, please let us know and tell us which camera, film, shooting conditions, what where you shooting, and whether you were using flash or not. That will help us help you. But rest assured AFS lenses can take a beating, so this should not be a problem.
Hope this helps,