They go bad. But be sure the USB port is not the problem. Plug something else into it and see if it works properly.
Recently I started my pc and my mouse didn't respond to my motions. It's an optical mouse. The next time I tried starting my pc it worked fine and now it has stopped working again. I have checked all the connections and they are tight. My pc is listed in my profile and I have windows xp home edition with sp2. I would appreciate any help!!