The user was deleted:
You can restore a user.
2. The user is in another Universe:
You can invite the user to your Universe. Above the user list in the "Administrator" window click "Find and invite user", enter the user's email and send him the invite. The user will receive an email message and a Cerebro client notification about being invited to your universe. To accept the invite, the user will need to log into the Cerebro client, click the invite pop-up (or the "Lightbulb" in the Cerebro menu, in the upper-left part of the main window) and accept. After the user accepts the invite, he will appear in the user list of your universe, and you will be able to assign him the necessary permissions.