When you open your class, do you see one of these two messages: "This class has ended" or "This class has ended. You can still make exercises, but your progress will not be stored".
This message shows up if the end date for the class has passed. If you want to extend the class, you can do this by going to the Teacher tab, Settings, and then "Start and end date".
You can additionally decide to limit students' access to the course material after the date has passed. If the access is limited, they won't be able to solve any more exercises or read any more theory pages.
Please note: If the end date of the class is reached, the access code expires. You will receive the following error: "Access code does not exist" in your LMS.