In the world of dreams the rules of reality do not necessarily apply. Each dream is unique and it's a direct link to the soul and spirit. Psychology provides its own theories on what dreams mean, but so did tradition and the study of dreams for thousands of years before any relevant science had developed.
Dream Meanings - Coffin
To dream of a coffin means distress and anxiety, which you will easily overcome. If you see yourself inside a coffin then you will have a long life and good health.