Daedalus,
Try all the usual things:
1. Update to latest sofware (10.4.6)
2. Repair Permissions with Disk Utility
You could consider downloading
usboverdrive which supports just about any USB mouse.
There were some issues with USB mice in version 10.4.2 (I think), but unless you have changed any system settings, I'm not sure why your mouse has stopped working.
Good luck.
AppleMacster