1. Check mouse is plugged in, if it is, unplug it and re-plug it back in. Reboot machine.
2. If Wireless, check the batteries first. Best way to determine if batteries need replaced is to look at the bottom of your mouse, if a small light is not on - replace the batteries.
3. If both 1 and 2 have been done, the drivers for the mouse need to be re-installed.