If the session is cancelled you can then restore this under the "Show Cancelled Sessions" button within the admin calendar page:

When clicking the "Show Cancelled Sessions" button, the calendar will then show you the cancelled sessions. In this case, we will click and restore the October 24th session from the calendar page:

There will be a pop-up on the left-hand side of the page that will show you the details of the session to be restored. Please click on the edit button as shown/highlighted below:

There will then be a pop-up on the right-hand side where you have the option to either edit the details of the event or restore this event. In this case, we will choose the option to "Restore this Event":

Please click on the hide cancelled sessions button to revert back to the main calendar:

And this will now show you the restored event/session on your calendar: