Apple wired mouse only scrolls up
This is a common issue with Apple Wired Mice (a.k.a. Mighty Mouse) products. There is no permanent fix, the issue happens when the mouse gets dirt from your fingers building up in the mechanism, and eventually jams.

You can fix it temporarily by following these instructions. There is a section "Magic Trackpad, Magic Mouse, Wireless Mighty Mouse, and Apple Wireless Mouse, Wired Apple Mouse, Wired Mighty Mouse", the instructions are in the second paragraph:


  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 might
  have collected on the internal hardware. Watch a QuickTime movie
  demonstration of this procedure. (The movie is approximately 600 KB.)