Hi [first name],
As you may already know, [insert problem and time period]. [Briefly explain expected outcome and actual outcome.]
It is completely unacceptable for [reiterate problem]. I’ve heard from customers who [three examples of how the problem affected your customers]; you depend on us to [goal of your product] and we hindered more than helped yesterday.
I want to apologize for the [problem] and, more importantly, I must apologize for the total failure to communicate this to you.
One of our company’s values is [core value or company mission] and we glaringly missed the mark yesterday. We are taking this incident very seriously and are making plans to ensure that it will never happen again.
To thank you for your patience, we will be giving [discount, coupon code, free item, etc. and time frame to use it]. And [something you’ve already done to make up for the problem: reimbursement, refund, etc].
If there is anything else our team can do to improve your experience, please don’t hesitate to reply to this email and ask!
[Your first name]