|Batteries tend to go bad slowly but once in a while maybe a single cell went bad or some connection broke inside. |
Even if it showed 100% charge that really doesn't mean much. You have to test it with a new known good battery to decide where the failure is. It may be that the battery is still under warranty so check that.
Batteries used to cost $300, less now. I know this will be expensive but consider donating it and getting a new system. At least compare costs of new versus battery.
Hang up and live.