Which battery are you referring to... the 12V battery or the high-voltage EV battery? I assume you mean the 12V battery?
I could easily imagine that leaving the car in drive (D) might keep some kind of electronics engaged, which could drain the 12V battery given long enough. At that point, the car will appear "dead", although the EV battery should be perfectly fine. The car, however, cannot function without a properly-charged 12V battery, which is used to power the electronics and do other key things such as engage high-voltage relays for charging and traction.
Hopefully, all you need to do is jump the car (yes, just jump it, the manual contains instructions) and your FFE will wake up again.