If you run out of time (18 months) attempts at any given subject (4) or sittings (6), you will be required to start your exams again. Before doing so you must do at least 10% extra study. This is assumed to amount to approximately 60 hours of study. What form this study takes is at the discretion of your school.
But before getting into all of that you should make sure that you are fully prepared for whatever attempts you currently have left.
Very few people ever have to start again.