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.