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Probably over-discharged. If that's the case, the battery is pretty much gone and will need a replacement. If you are the handy kind of guy, you can find a suitable replacement on eBay (Li-ion 300mAh, 30mm x 20mm, usually under $10) and only requires a little bit of tinkering. The case does have a tendency to crack under force so you need to be careful or prepare to glue it back. If you are not, better contact digiZoid and see if they can repair it for you.