Don't know if this'll work on an Apple, but I sometimes have that happen on a pc with optical mouse, mouses, mices.....
If it has a usb connection, simply unplugging and replugging gets it to work.
When it's a a ps2 connection, I usually have to reboot.
These are all things to try before concluding the mouse has gone bad.
Then..... if it's a ball mouse, try cleaning the ball and the roller bearings it rides against using rubbing alcohol and a q-tip.
Test the mouse on another system if you can. Test another mouse on your system. These are ascending levels of trouble-shooting that help isolate the trouble spot. If something works, stop there. If it doesn't, then go on to the next thing.
Hope it's solved by the time you read this! Good luck.