My next [specify] payment will be due on [DATE]
Due to an unexpected emergency that has occurred, I will be unable to make this payment by the [date] I am requesting an extension of [time] to make this payment.
If you review my file, I believe you will find that my payments have generally been made in a timely fashion.
My relationship with your bank is very important to me, as well as my desire to maintain a good credit rating.
Thank you very much for your consideration of this request.