Before you take the lens apart. Does this lens have an "A" position on the aperture ring?
If yes, is the aperture set to "A"? and you should be able to use all exposure modes.
If no, Go to the custom menu, and make sure the item "Allow aperture other than A" ( or something like that) is set to yes. You can leave this setting for all lenses.
for use on the camera:
- Set mode to M
- Set your aperture using the aperture ring.
- Push the Green button, the camera should set the shutter speed.
If this does not work than the lens may in fact be broken.
One other thought. Remove the lens from the camera. Set the aperture to some middle value, it doesn't matter what. Move the aperture actuation lever on the mount. You should be able to see the aperture blades closing and opening as you move the lever. If the blades don't move, the lens definitely is broken.
Hope this helps a bit.