If you're unable to locate a user in your portal, they may have be 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 the response next to 'Activated' (equals) from 'Yes' to 'No', then click 'Filter'. Optionally use the 'Username', 'Email', 'First Name' and/or 'Last Name' text boxes to further narrow down your filter.
A list of deactivated users appears.
If the user(s) appear in the list, tick the checkbox beside their name(s), click the 'Bulk Action...' button and select 'Reactivate people'.
It's also possible to reactivate users from their profile directly. Click the user's name in the list to open their profile and, in the 'Access' section, click the 'Reactivate' link.
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, regretfully, the account (including it's history) cannot be retrieved and a new account would 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.