unable to connect to database

[eluser]Unknown[/eluser]
i am using the following function to initialize db:

Code:

$this->load->database();

application\config\ files well defined with connection strings

when i execute:

Code:

$this->db->insert('blog', $data);

i get an error msg:

A PHP Error was encountered
Severity: Notice

Message: Undefined property: Blog::$db

Filename: controllers/blog.php

Line Number: 75

Fatal error: Call to a member function insert() on a non-object in D:\1wwwroot\system\application\controllers\blog.php on line 75

what should i do
thanks in advance