If you're syncing your classes through Clever's rostering service, you will still have the ability to unarchive classes by following the steps in this article. However, if the class has been removed from your Clever roster, you will only be able to unarchive the class for 24 to 48 hours. Once Clever and NoRedInk sync, those classes will automatically re-archive.
To see if an archived class has been removed from your roster in Clever, you can navigate to the Archived Classes section of your Manage Classes page and hover over the question mark next to the Unarchive class option.