First try a new mouse. You're due for an upgrade anyway. Don't worry about that ball any more, you can get an optical mouse for about $5

Newegg.com - $0 - $10, Mouse, Input Devices, Computer Hardware

Did it lock up or were you just not able to click on anything. In the future you can chck this by trying a key, caps lock (light should turn off or on) or Windows key (next to space) should open the start menu.

If you have been having problems, did you notice them soon after a software install or system update?