When the Server Config exits, it checks for at least one Master User. If it does not find one, it will prompt for one.
Under normal circumstances it would be difficult to lose all of the Master Users, as the Accounts Configuration will not allow you to save without at least one Master User.
If you have lost an employee or contractor that was the sole Master User, you can edit the Accounts.CONF file and set one of the users to e-mail=1.