A similar thing happened with my wife's S-Max. Parked up fine in the evening, come the morning the car wouldn't start, no power at all in the car. Got the RAC out and they charged it up, couldn't say what it was as the diagnostic machine drew a blank. Took it to Fords who initially did the same but after it happened again, they had a proper look at it and diagnosed a problem with a clutch switch that kept a constant drain on the battery. I think it started with the Stop-Start failing which as it was summer was a bit more obvious than it would be in this weather!
This was when her car was only a few months old so took the shine off a bit.
May not be the same thing at all, but it took some thorough investigating and perseverance to get it sorted.
