If the corresponding button is no longer visible, it is possible that the logout button has been deactivated for the respective employee group.
Check your settings for this.
Switch to the admin area and go to Settings. Then click on the Users button.
Scroll down until you find the item Hide logout button. Click on the pencil icon.
Check whether a tick has been placed next to a group. In order for the logout button to be visible to everyone, NO tick must be set.