Glad to know the batteries are working again, please take good care of the batteries per the manual here: https://dl.djicdn.com/downloads/ ... tery+Safety+Guides+(EN).pdf, especially do not leave it alone for a long time without being charged or discharged.
My guess was that the batteries slipped into Hibernation mode.
You just have to connect the battery to the charger and after a period of time (some time coding for charging voltage present), it will come out of hibernation.