Something similar happened to me on my first trip to the 'Ring. Was on my way home by myself, vibration in the wheel all the way from Germany, 10km from Calais it was getting really bad... pulled over at the last service station before the Tunnel, was pulling in to park, the car fell forward!
Jacked it up and got the wheel off with the help of a Frenchman, put the spare on... there were only two wheel bolts left, of which one had sheared. The wheel was only held on by one bolt!
Never found what it was in the end, whether or not someone had tried stealing the wheel or whether my tyre place didn't torque the bolts correctly...
But it's a dangerous thing and I'm glad you're alright. For this to happen at any speed is potentially catastrophic. First thing I'd check is whether or not the dealer had torqued the bolts. Were you able to recover any of them?
Also, I'd be inclined to want a new car... with my incident it was quite lucky, the car fell forward at almost no speed onto the wheel and was held up by the wheel/wheelarch interface. With this, no telling what damage could have been caused without thoroughly inspecting the vehicle. Also there would have been undue stresses placed on areas of the car not designed for it... might be better to get a new one, if you can.