Subject: Our apology for What went wrong, in a few words
Dear Customer's name,
On behalf of everyone at Your business, I am sorry for What went wrong, in a few words. You should not have had to deal with this.
What happened
What you have done so far
What you offer
To stop this happening again, How you will stop it happening again.
If there is anything else we can do, please reply to this email. Thank you for your patience, and for giving us the chance to put this right.
Kind regards,
Your name
Your business