Backups may have stopped, and a "grace period has expired" error message displayed on the Core GUI.
The licensing section of the Core shows a "failed to send heartbeat of the core" notification.
One possible cause for this, is that the core ID has been changed.
While the Core machine may have a new ID, the license portal is still associated with the old core ID.
Delete the Core from the license portal.
Once deleted, go to the core machine itself, and go to the licensing section of the configuration tab, and click on the "contact now" button.
This will update the License Portal with the new core ID, and the License Portal will display the core again.