The battery is probably just fried, rechargable batteries typically have a life of 1000 charges before they start to get weak. You can just replace with any rechargable 3V battery. Look in somewhere like Maplin for what you need.

If you can't find a cell one like that, then get 2 rechargable AAA's and wire them to the same place. Just make sure the +ve and ground are going to the right places.