Can't acccess personnal library

Usually, collecting data from your database is done by a Model, not a Library.
In a model, you can use the $this->db object without instantiating the super-object.

One more tip about your method. You are looping through a result_array(), but it seems that you expect the result to be 1 record.
In that case you can return this record like this:

PHP Code:

$query = $this->CI->db->get_where('cms', array('page_name' => $pageName), 1);
return $query->row_array();