Firstly, we have to listen what customer need.
Let they say all things with sympathize and apologize them. Then tell them what we can do at that moment, maybe offer them the service which similar with their need, and inform the problem to leader, consult them what we can do and ask them to decide what we can do next. And i will tell customer that next time we will try to protect this problem not to happen again, and thank you them to choose our service.