This means that the number of users registered in your Universe has exceeded the number of licenses you have. Or your license has expired.
In these cases, Cerebro enters Read-only mode.
To check the number of licenses, open the Administration screen: Main menu > Tools > Administrator..., Universe tab. In the lower-left part of the Universe tab, you will find the maximum and the current number of users. The maximum number of users being 0 means that your license has expired.
The current number of users highlighted in red means that the maximum number of standard users allowed is exceeded. You can delete users and, if you know their email, restore them at any time. Another option is to purchase more licenses.
To purchase additional accounts and renew licenses, use the User account on the website: https://cerebrohq.com/en/ac-login-page/.