Simply replace the unfilled areas such as "__" and "_[date]_" with your information.
I have received your notice of [date] showing my account past due by [number] days in the amount of $ [amount].
I apologize for my late payment. My family has recently encountered some unexpected financial setbacks. We have incurred [type of expense] expenses that have hindered our ability to pay our bills. It is my intention to fulfill my financial obligation to you. I am humbly asking you to work with me to establish a payment arrangement. I am enclosing a payment of $[amount] as a good faith gesture.
I will contact you this [week/month] so that we may reach an agreement in the matter.