Dear …
RE: Missed deadline - __(Name of Project)__
On behalf of __(company name)__, please allow me to extend our deepest apologies for not meeting our agreed deadline on the __(name of project/campaign) __. We take pride in our professionalism and our ability to meet deadlines and when we fall short of that mark, it is of great concern to us.
As you know, we experienced a severe interruption from __(describe situation)__ that made it extremely difficult for us to stay on schedule. It is our intent to do what we can, however, to make up for this setback. We have now committed our full resources to this project and are very confident that it will be finished quickly and without further hindrance.
Thank you for your patience and understanding in this matter. We are grateful for your willingness to continue working with us and look forward to the future of a long mutually rewarding relationship.
Yours truly,