2015 Subaru Foresterengine shut off

Admittedly, for about 30 minutes this morning I sat
in my car with the key turned to the on position (so I
could charge my phone and stay warm) while I
waited for someone. When I started the car, we
drove a very short distance to his car. While we
were driving, the car was beeping incessantly. I
thought this was the seatbelt warning, although I
thought it was uncommonly  loud and insistent.
When we got to his car, I put the car in park and
turned it off. Then I tried to restart it... nothing. No
lights, sounds... dead. I’m hoping it’s just the
battery... any insight?( I also thought that iI saw a
symbol flashing, but when I looked the symbol up, it
was the power steering light. )