When a class is deleted, it will not longer show in your calendar.
If the class was deleted before the time it was scheduled to begin at, all members will be cancelled from the class. If it is cancelled after the class has begun. members will stay booked in.
To find members who had been booked into a deleted class ,if you know of one member who was booked into the class you can click on the booking in the activity on their page. Then you have the list in the cancelled sectioned. If you click edit details of the class and then save , it reactivates the class but the people will need to be moved back from the cancelled list to the attending one.
If you do not know any members who had been booked into the class, you check the push notification logs in Connect > Push Notifications to find users who have received a cancelled class notification for that event. This is easier if you know a rough date and time it was cancelled.