An apology letter can be written for business relationships, or personal relationships. It is a good idea when face to face is just impossible. There is a good example of a personal apology letter at: http://www.sarasota.k12.fl.us/bhs/bryan/bryan_apology2.html. If you are looking more for business letter examples, http://www.perfectapology.com/sample-business-apology-letter.html might work well
. for you.