I have had the MMI crash a couple of times in my previous car. Sometimes when I've been doing something like as simple as changing radio stations, I've had the screen freeze and the MMI screen goes blank. It takes some time to boot back up, like it is resetting itself.
I've also had it once before with the ignition off, I could not get the screen to go down. No power on the screen, just blank and it would not hide away.
I wouldn't be too concerned, I'd say there has just been a clash in the program like with mine. It has also happened to my granddads car, where he couldn't get the screen to come up on one occasion. When I looked the next day, it worked fine.
If it happens again within a short period of time, take it in but otherwise I doubt it will happen again.