Stay cool and focus on their plight. If you can focus on and meet their needs, they'll be singing your praises in no time. Get to the bottom of those complaints. Be a Problem-Solver. This will earn you a reputation for making things right again.
Folks love doing business with someone who gets it right.
Even if it means bending over backwards, do it. In the long run, your efforts will be worth it. Be a Quick Thinker and Schmooz. You not only have to solve the problem, but you need to do it quickly. Find the best possible answer to the problems and carry out the solutions promptly.