It can be something in the car.

But I bought a new battery in 2016. Drove the car about every three weeks, one time, about 3-4 months after I bought the battery - I jump in the car (garaged so it was not locked) and NOTHING. The battery was around 3 volts.
The store exchanged the battery. Guy said, "Yea we see about one every six months that does that - no idea why."

If the battery refuses to charge, well, it died.

If the battery charges right back up - it is likely you have excessive current flowing.