It sounds like the problem is either the battery itself or the metal contacts inside where you put the battery have become corroded. To replace the battery, you have to take the device apart, which can be tricky. If you buy a replacement battery online, most of the time you will receive instructions on how to perform the replacement yourself. If you don't feel comfortable doing the replacement yourself, then check with some of your local geek shops and see if that's a service they offer.