You can un-cancel an individual class instance but you can not un-cancel a series of canceled classes.
To un-cancel an individual class:
Select Schedule
Go to the date the class was scheduled
If you do not see the class, at the top of the page check the box Show canceled classes
Select the three dots to the far right of the canceled class
Choose un-cancel
Select Show on schedule or Hide from schedule
Un-canceling a class does not reinstate the clients who were enrolled in the class nor is there a way to see a list of clients who were removed from the class.