The notifications contain links to reviews and topics that may no longer be part of an active review. If you click these links, the Review screen will refresh and will either open the topic or display a message that the review is no longer available. Because of the current status of the topic or review, the ribbon menu and screen layout will be updated, and the Table of Contents pane will be hidden.
Showing the Table of Contents Pane
If this occurs, reopen the pane by clicking the Show/Hide button in the left side of the window. Then select a current review or select the link to the News Feed page.