Another student used my completed work, what should I do?
If I understand correctly, the other student took the class after you took it, so it's clear that she cheated from you, not vice versa. I suspect they merely want to talk to you to find out exactly what happened. You are probably not in any trouble, unless there is a school policy against sharing old assignments, or the professor asked students not to do this, or you knew that she would have the same assignment when she took the class. So I would simply answer any questions they have, and explain why you did what you did.