If you've just cancelled a class and want to un-cancel it, you can restore the class. Before doing this its worth noting that previous signs ups will not be re-instated. The class will just be restored with all previous details.
To do this, click Classes > Schedule > Filters (light blue button off to the right)
Click box titled Show Canceled > then Apply filter.
Click the dropdown menu to the right of the cancelled class > choose 'uncancel.'