Inactive or Blocked users are those users who were blocked by the admin from accessing the portal. If user creation was done via CSV import and the importer put '0' in the Status column, that user will be uploaded as inactive.
How to view Inactive Users
Inactive users do not appear by default in the user management reports because the default filter only shows active users.
In order to view the inactive users, you need to change the filter from 'Active' to 'Inactive'
Go to Administer
Change the Status value from 'Active' to Inactive'
How to Re-activate an Inactive/Blocked User
View inactive users by following the procedure above
Hover your mouse at the end of the user's name
Click the small arrow
Here's the quick video guide.