The mouse doesn't work?
Check to make sure the mouse is still properly connected to the computer. If the mouse has been disconnected, you may have to reboot your computer.
Like the keyboard, the mouse pad is sitting in the open most of the time getting dusty, wet, slimed, or anything else that happens on your desktop. The mouse (if you are using the kind with the rolling ball insert...an infrared mouse would be immune to this) then rolls over whatever has collected on the mouse pad and gets inside, gumming up the works. To clean the mouse pad, wipe it off occasionally with a damp cloth or get a new one.
You also need to clean your mouse regularly, as often as twice a week. If you turn your mouse over, you’ll notice a round ball with a cover over it. This cover can be twisted off and the ball will come out. Roll the ball on a clean, lint free cloth. Then take a look at the rollers inside the mouse. Take tweezers, a screwdriver, or even your fingernail to scratch the dirt and lint off the rods. Next, you should look inside the mouse and clean out any other dirt or lint that is hiding in there. Finally, replace the ball and twist back on the cover.