Dear […. ],
Thank you for your letter alerting us to the problem you have been having with our [Your product/service]!
I am sorry you have been subjected to such a frustrating series of events. What you have experienced is inexcusable.
I have [Explain what you are going to do to solve the problem]. I extend my own apologies for the inconvenience this problem has caused you. It is apparent that we need to [Actions you will take to avoid such mistakes happening again].
Please accept my sincere apologies. We will make every effort to see that this never happens again. We appreciate your business and will do everything we can to serve your needs.
With Best Regards,