If an Apple device, odds are it's quit working, do you have a cheap USB Scroll wheel mouse to test with?
Is it a Mighty Mouse?
If the scroll ball on your Mighty Mouse has become discolored or dirty, use a clean, lint-free cloth lightly moistened with water to clean it. Wipe the ball and the surrounding area, making sure to rotate the ball itself to ensure complete coverage. If the scrolling feels rough or if the scroll ball isn't scrolling up, down, or side-to-side, hold the Mighty Mouse upside down and roll the ball vigorously while cleaning it to help dislodge any particles that may have collected on the internal hardware. Watch a QuickTime movie demonstration of this procedure. (The movie is approximately 600 KB.)
Thank you so much. I really appreciate your answering me. You did solve the question I asked and I can now scroll down, but not up. I can't zoom using the option key, which I checked off on my sytem preference. I have Mouse Model A1152 -- three years old, plump, and not cool but I like the way it feels in my palm.
I am going to try to get a cheap USB scroll wheel mouse.