problem

Hello , i ve a big problem and i want you to advice me. Yesterday oyr clients come a stay in our apartment. Today the decide to live without any surios reason and even the rheir reserve started 6/07 until 18/07 today they cancel in the platform. They i asked fro me to sent money back. So my question is what i ve to do? and How i can give them money back? i lost so many days  . please help me thank you