Tranny Question 4 friend

My friend has a 1992 400E with 150K miles. His tranny just stopped working! Yup that's right, just stopped working. It was fine, and he parked it in his driveway. Three days later he started it and put it in gear and it wouldn't go anywhere. No problems or anything out of the norm before that. Car is regularly serviced. What could have happened? It was driving perfectly when he parked it and just would not run that day when he started it.
Thanks