Yes that’s the problem. Your cell voltages have gone too low. If you look at this page:
we talk about the voltage ranges. At this point we can’t make specific recommendations, but I will mention there are numerous videos online showing how to recover a battery that has gone too low. I would advise to do it under supervision and make sure you do not leave it unattended while trying to recover it. Also, make sure you charge it while in the storage bag.
I would also recommend purchasing a new battery as a fallback measure. I’d be surprised if you’d get the same amount of flight time anymore now that the battery has sustained some level of damage.