If you're unable to locate a user in your portal, they may have been deactivated. This means their details still exist in your portal, however, they currently don't have access.
To confirm that the user has in fact been deactivated:
Navigate to Admin → People and click the 'All People' button.
In the window that appears, change Activated from Yes to No.
If relevant, apply additional filters (such as the person’s name or email).
Click Filter. The window will close, returning you to the table of people.
Click Apply Filters to finish applying your filters.
If the user appears in the list, tick the checkbox beside their name, then click Bulk Actions and select Reactivate People.
To reactivate from a user's profile instead: Click the user's name to open their profile, then in the Access section, click Reactivate.
Please note: If the user does not appear in the list, it's possible their details differ to what you're searching for, or their account has been deleted.
In the case of the former, please verify you have the correct details (i.e. username, email address, and so forth). In the case of the latter, the account (including its history) cannot be retrieved, and a new account will need to be created.
Confirming whether a user's account has been deleted
To confirm whether a user's account has been deleted, use the 'Timelines' report.
This report can be accessed at Admin → Analytics, and allows you to filter by various actions including 'Deleted' which returns anything, including users, who have been deleted from your portal as well as who performed the action and when.
Note: If the list is extensive, you could try searching the browser page for a keyword such as the user's name or email address.
Similarly, you can search using the 'Deactivated' action to confirm users whose accounts have been deactivated along with when and by whom.