Well, some questions might help us to help you.
Is this lens new to you?
If not, did it work before? If it has stopped working has it had any knocks, bangs or other physical contact?
What cameras are you using the lens on?
All things being equal, if it was working before and you are using it on an APS-C DSLR, then my first reaction is that the aperture blades have stuck. Normally, with a DSLR, you look through the viewfinder and you see the image with the aperture fully open, it only closes the aperture to your selected setting to shoot or if you choose exposure preview.
Given the age of the lens, if that is the case, it is extremely unlikely that it will be serviceable - it's way out of service life. That lens was replaced in 2004 by the rather better 15-85, so you might pick up one of those.
cheers, TREVOR
The mark of good photographer is less what they hold in their hand, it's more what they hold in their head;
"All the variety, all the charm, all the beauty of life is made up of light and shadow", Leo Tolstoy;
"Skill in photography is acquired by practice and not by purchase" Percy W. Harris
