Oh man... Unless you had left the lights on or some other drain, I would have taken it directly back to the closest desk of that company and told them to swap the car. A flat battery is usually either from an electrical drain (like leaving the lights on) or from the charging system not really charging the battery any more.

(I did have the exception a couple of months ago--the original battery from our 2015 CRV decided not to start the car any more. It was way under "empty" voltage, and it was probably 10 years old, so I decided to just swap it and see. No problems with the new battery.)