Check the simple things first..........without wanting to sound too obvious, you were looking at changing a headlight bulb..............did you leave the lights on when you left the car, or maybe a door slightly ajar. Can't think that if you have had no warning signs, the battery would go completely flat just like that. Our experience shows that if the battery has gone completely flat, then a slave battery won't start your S, the battery will need to be charged first.
Needless to say, don't have the slave car running when you jump start, and make sure the two cars aren't touching.
Might be stating obvious, but often the simple things are the answer. Or as the meerkats would say "Seemples".
Good luck