Simply replace the unfilled areas such as "__" and "_[date]_" with your information.
Dear [Mr./Ms./Mrs.] [last name of customer],
Thank you for your [correspondence, phone call] of [date of contact] regarding your account. I would like to profusely apologize for the treatment you received from our representative. This is not the type of service that [company] condones, and we have taken action regarding this instance.
We value all of our customers, and hope to be able to serve you in the future. We hope that you will allow us to show you the true customer service of [company], and to make up for this disappointing experience.