In case of an emergency, ask questions
Whenever you start to feel stressed and don't know what to say to a customer, start asking questions. Questions are your emergency kit for all situations.
Source: Sales consultant/trainer Steve Waterhouse
Simply asking something like, "Please tell me a little more about that?" or "What could we do to help you?" will often turn it all around.
Once the prospect is talking, you'll have the opportunity to regroup and prepare your next move.