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