Or maybe your mouse is all gunky inside. the first thing you do in that case is remove the ball and clean it then find the rollers, there should be three of them, and scrape them with a wood or plastic toothpick or something similar to get all the gunk off. Don't use anything made of metal. You'll have to rotate the rollers while you scrape to clean them all the way around. Then you have to get inside. The screws may be hidden under a label that you'll have to poke through to access them. Some mice have clear plastic slides that elevate the mouse slightly above the surface. If there is a screw under one of those you'll have to remove it and reattach it later but don't use anything too strong to reattach it or you'll never get it off again. Once the mouse is open you'll be amazed at the amount of brown slimy gunk, hair, crumbs, etc that can build up inside. Clear out any dirt you find, close it back up and then if it still doesn't work it's probably time for a new mouse.