Act Fast.

A service provider ran into trouble.  They had problems with their equipment and their people, and it impacted their customers directly.  But only after several months and hundreds of customer defections did they communicate with their remaining customers.  No company is perfect.  Yours will make mistakes.  But when it happens, respond quickly and professionally.  Own the problem and offer the solution.  The goodwill you gain will make customers more loyal than they were before you dropped the ball.