The most time and resources should be spent on your current client base, but if you have lost a client, it generally takes fewer resources to try to win that client back than it does to try to find new clients. The reason your client left may be a minor misunderstanding, an easily remedied problem, unsatisfactory results or simply hurt feelings. Your client may have left abruptly or may simply be drifting away and contacting you less for your services.
Remain patient, courteous and understanding if a client is angry and confronts you. Ask how you can improve and don't give excuses. Be empathetic and try to see the situation from the client's perspective no matter how absurd it may seem.
Ascertain the reason your client left or is drifting away. If you are uncertain of the reason, invite him to lunch to discuss why he left or why there is a reduction in project flow. The problem may be a simple misunderstanding requiring a simple apology, a perceived flaw on your part that requires improvement, or the client may be unsatisfied with results. It is also possible that he simply doesn't have the funding to continue using your services; in this case, consider offering flexible credit terms.
Send your client a gift to tell her she is a valued customer and offer amends for your unsatisfactory service. Include a letter or write a follow-up note to admit your mistake explicitly and explain what you have done to rectify the situation, if applicable. Or send the gift or a friendly email simply to tell her you enjoyed doing business with her. Consider offering your services at a reduced rate for a limited time if she comes back.
Create a quality assurance program. If your client accepts your invitation and returns, ask him for feedback on the services you performed. This lets him know you are trying to improve and keeps communication open so future misunderstandings don't occur.