1. Check the connection of the mouse cable in the back of the computer and make sure it is tightly plugged in.  If not, plug it in.
2. Once you are sure that the mouse is properly plugged into the back of the computer, power off the computer and then restart it.

The mouse works, but it is difficult to use (it doesn't go where you want it to)

The mouse may be dirty. Remove the mouse ball cover, and then remove the mouse ball.  Remove any dust or dirt that may be inside the mouse and then replace the cover.  (The cover should twist off and then the mouse ball can be removed.)