My Phone disconnected itself from the car a couple of weeks ago.
Numerous attempts to reconnect on my drive failed as the car was not found from my phone. The phone was showing numerous blue tooth devices within range, but not the car.
I tried everything in the book (and the internet) still no luck.
I eventually went to the main dealer who said, show me what you are doing. I did the same thing I had been doing for two weeks and it connected immediately
Evidently the main dealers have the same problem if they try to connect a phone to BT in the showroom, it connects to so many and then gives up.
So, the advice if a disconnection happens again, is to drive somewhere away from other BT devices and reconnect.