It happens sometimes and life can get crazy. As long as you cancel 8 hours prior to the start time of your class, your class booking will be added back to your available class package. This is considered an "early cancel".
If you do not cancel within this time frame, your class will be charged to your account. This is considered a "late cancel".
If the class you want to take is already full, you will be added to the Wait List for that particular class. If there is an early cancellation, the first person on the wait list will be added to the class in the order they signed up for the class. If you are added to the class, you will receive notification via text message verifying that you have been added to the class.
If the class remains full and you do not get into that particular class, you will not be charged for that ride. You will receive that credit back toward the next class you sign-up for.