My mouse has stopped working why

what sort of mouse is it? is it a wireless/ infrared mouse (i.e., with no cable connecting it to the computer) or is it a cabled mouse?

(a) wireless or infrared? the battery inside the mouse may need replacement or the receiver unit might not be plugged into the computer correctly.

(b) cabled? the cable may be faulty or the connector that plugs into the back of the pc may be damaged or disconnected. or you may have plugged it into the wrong socket on the computer.