I would have to believe that the coupler has slipped. On the couplers that are not keyed, if you over tighten the clamp screw you distort the hole and reduce the holding power and slippage can occur.
Can you jog the machine back to the C/S makers and do a cold start and have the makers aligned correctly? If so then the couplers did not slip.
Alternatives would be the motor resolver is loose and rotated (doubtful) or that you experienced an electronic issue.