Apology for Late Reply

Introduction:
I wanted to reach out and apologize for the delay in my response to your email. I understand that my delay may have caused inconvenience and I am truly sorry for that. I would like to explain that I was out of office and had a heavy workload upon my return.
Body:
I understand the importance of your message and the impact of my delay on your work. I want to assure you that I am committed to providing prompt and thorough responses to all of my emails. I have reviewed your email and I have an update on the status of the matter. I would like to schedule a call or meeting with you to discuss it further if necessary.
Closing:
Once again, I apologize for the delay and any inconvenience caused. If you have any further questions or concerns please don't hesitate to contact me.
Sincerely,
[Your Name]
It's important to remember that when apologizing it's important to be sincere, take responsibility and provide a solution or next steps to the situation.