Explain the situation to italki and the teacher. Technically you booked the lesson and should pay for it. The teacher missed the opportunity to teach somebody else, so they may feel that they still want to be paid. Imagine that you booked a plane or theatre ticket for the wrong day...often you still have to pay. But if it is a genuine mistake the teacher might agree to reschedule the lesson. Of course the problem might be that they work completely different hours to when you are awake, and it might not be possible to reschedule.