If you cancel a class on the calendar and later wishes to reactivate it, here are the steps to do so.
1) The below image shows a class canceled on the calendar. (Shown in red).
2) Click the canceled class on the calendar and select 'MODIFY CLASS'
3) In the next pop up select ' SELECT RECURRING CLASS' and click OK
4) On the next page, shown below, change the class status to 'ACTIVE' from 'CANCELED' and click Submit.
This will change the class back to being active once again on the calendar.