No, there shouldn't be a comma there. This is how you should punctuate your message: "Sorry I haven't written for so long. I was very busy with revising for my exams." There is no comma because the first sentence is a shortened version of 'I'm sorry that I haven't written for so long'. If the rest of the sentence explains WHAT you are sorry about, you shouldn't use a comma. If the rest of the sentence is simply an explanation, you need some form of punctuation after 'Sorry': "You haven't written in ages!" "Sorry! I've been busy revising for my exams."