The best way to handle such a situation is to give customer the confidence that his/her problem will be listened to. Afterwards, let the customer speak and hear him/her out. Try to understand the process and see if customer's expectations are right or not. It is important that customer let's it out, even if you have understood the problem, ask customer for sometime. That will give customer the confidence that his/her problem is being given attention. Later, explain the process to the customer and it is best to come up with a couple of options for the customer.