When an account has been deactivated due to an expiry date being set on the account the user is not listed in the user management tab. To list user accounts that have expired, change the filter option "Active" to No and "Apply" the filter.
Locate the user account to re-activate and click "edit" in the corresponding row.
Scroll down to the bottom of the user profile edit page and remove the expiry date, leaving it blank or set it to a future date if you would like the account to expire at a known future date.
Press "Save" to apply the change and the user account should now be listed as an active user account.