Yes, I have lost small and large clients before. It is part of being in a client-services/sales role. You do everything in your power to a) prevent this difficult situation and b) remedy the situation if you are in it. Change the focus from pure price. Find out if the customer's needs have changed over time (what did you miss?). Find out if there is a different decision maker you can talk to. Find out what you can do to qualify for the business at a later date (enhance the product, provide better service, etc.). Learn from the experience and apply those insights proactively to other clients. Then turn around and go win new business.