ross, the very first thing you want to do is to clean the battery contacts. Make sure they have not corroded to badly. If those are okay, you will have to disassemble your GB and take a look at the components. You could try and clean it with some isopropyl alcohol and carefully check to see if there is anything obviously corroded etc. But until you have it apart, anything will only be a guess. Use [http://www.ifixit.com/Device/Game_Boy#guideList|these guides] to help you with taking it apart. Hope this helps, good luck.