The directory user form doesn’t allow the administrator to unarchive a user.
The workaround is to directly update the database table:
In the database, Table USERS, when a user is archived his username is prefixed by “ARCHIVED_” , “ARCHIVE” field is set to “Y”, “ACTIVE” field is set to “N”.
To set the user active again you have to remove the username prefix, set the “ARCHIVE” field to “N”, “DATE_ARCHIVE” to NULL and “ACTIVE” field to “Y”.