Dear Head of Admissions (Name)
I am sorry for being unable to attend college (reasons)
Let me assure you (reasons why you want to return and assurances that past circumstance/predicaments won't happen again)
I hope I can persuade you (reasons why you would be good for the course and your objectives and future goals)
Please give me this second chance to prove to myself and the college that I can become a worthwhile and achieving student.
Thank you for taking the time to read and consider my appeal.
I look forward to hearing from you.
Please remember to print the letter out and sign in your own handwriting
and post by snail mail preferably with proof of postage by means of signature.
Don't be tempted to email it and remember to spell check everything and paragraph properly.
Try and make the letter no larger than one side of a4 paper.