A large number of things in our house are battery operated. We have a slew of animated holiday decorations, toys, electronics and appliances that all require batteries. Let's not forget the smoke detectors in our house as well. One of the ways we've kept all of them in good working order is to change the batteries as soon as they die. Anything that is left sitting with batteries in it can develop corrosion that could ruin the item.
Corrosion is often a whitish, crusty stuff that forms around a battery terminal and anything touching it. I recommend wearing rubber gloves just to be safe when removing the batteries just in case any of the battery acid has leaked out. Dispose of batteries properly.
Begin by taking a paper towel and wiping the battery case out gently. This will remove any moisture. As for the metal parts, which is usually the part covered by the corrosion, using a small piece of sand paper or an emery board (like the ones you shape fingernails with), gently sand the corrosion off. Wipe the area out again with a new paper towel. You may also want to blow the inside of the battery case out with canned air or another dry-air source to remove any dust and moisture.
Place fresh batteries in the case and try the device again. Wash your hands and gloves thoroughly afterwards.