Apple's Mighty Mouse is great. Unfortunately, it's not uncommon for the mouse to stop scrolling. Often it will scroll up, but it will not scroll down. The problem is typically caused by dirt in the ball mechanism.
First, unplug the mouse from the computer. Then, either use an alcohol swab to rub on the little ball at the top of the mouse, or very vigorously roll it around with your finger in every direction while pushing it down. It will eventually start working again.
You could also try putting the Mighty Mouse upside down on a clean sheet of paper and gently roll the trackball on the paper to clean it.
Pro Support clients, please contact us with any questions or concerns.