One date booked by two customers

We had a situation where we had one reservation, from 14 August, and another customer made a reservation untill 15 August. We asked the second customer to change the reservation because we don't know what happend and why the date 15 August is double booked. 

She found us on our site and asked us what was the reason for that. We didn't know the answer and she cancelled the reservation.

What to do in this situations