Maybe you were ill, preventing you from submitting and/or attending the exam.
Maybe you did not deliver on your mandatory assignments, meaning your course manager gave you the grade NA (not approved).
Or you chose to not hand in or attend due to other reasons of your own. In either scenario you have a re-exam. Keep reading for all the details.
Remember you need to be sure if you are registered for the re-exam, see here for info on how registration for re-exams run.