When you store a lipo battery, rather it be in a device or not, you NEED to make sure it's around the nominal pack voltage. 3.7V on a single cell, 7.4V on a dual cell, 11.V on a three cell, 14.8V on a four cell, etc. If it's below or (especially) above this voltage it'll develop lumps. I won't go into confusing engineer details on why this happens, but it can cause all sorts of problems. The pack may not take a charge ever again, the pack may swell and go into thermal runaway on you during use, the pack could even catch on fire. Never store a lipo battery for longer than a week that isn't sitting at its nominal voltage charge capacity, it's dangerous.
I'm not saying you didn't do this, you may have. But just in case you didn't, I'd take the battery out and thoroughly check it for lumps or dents to make sure it's still safe to even have in your house.