In User profile sections, the User's Business details and Private details are separated into two panels. The business details cover all the public information that can be viewed by other Users, whereas the Private details are secured behind a Permission, so that those who do not have the Permission to view them will be unable to.
If the Private tab is greyed out and you're unable to open it, then you do not have the Permission to view the User's private details.
This permission can be changed by an Administrator who has the ability to change other Users' permissions, by going to Admin > All Users > select the Edit arrow for the User who requires the permission > scroll down to User Permissions > select 'Able to View Users' checkbox > Save.
You can also enable i for a User Group by going to Admin > User Groups > select the Edit Arrow for the Group > select 'View Users' checkbox > Submit.
