There are several reasons why a computer mouse doesn't work, including a bad USB port, a hardware conflict or a frozen computer. Try ruling these things out first as they are the most common causes of a mouse not working.
Computer mice connected to a PS/2 port cannot be removed while the computer is running, so be sure to shut down the computer before trying to diagnose a problem with a PS/2 mouse. Shut down the computer, remove the PS/2 mouse and reinstall.
A USB mouse that once worked but no longer functions should be unplugged and moved to another USB port on the computer.
Check a wireless mouse to determine that it still has power. Many wireless mice today have on/off switches located somewhere on the unit, so be sure the batteries are good and check that the receiver is within range. If all else fails, disconnect the USB adapter and plug it into another USB port.