Open Google Classroom
Go to Google Classroom.
Log in with your Google account.
Open Archived Classes
In the Google Classroom main screen, click on the three horizontal lines (menu icon) in the top left corner.
From the dropdown menu, select Archived classes.
Unarchive the Class
Find the class you want to unarchive and click on the three vertical dots (more options) on the class card.
Select Restore from the dropdown menu.
Confirm the Unarchived Class
The class will now be moved back to your active classes. You can find it on the main Google Classroom page.