From my understanding of your statements, it sounds to me like the nut holding the output flange to the output shaft has come loose and the flange has backed off far enough to disengage the splines or to strip them. The question as to damage will hinge on whether the output shaft can be reused. If it can the problem is minor.
BTW I have seen these nuts loosen but usually they cause symptoms way before getting separated. I have never seen one cause total uncoupling.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician