Finally, I found the solution to my problem.
I looked into the Magento system log file (var/log/system.log). There I saw the exact error.
The error is as below:-
Recoverable Error: Argument 1 passed
to Mage_Core_Model_Store::setWebsite()
must be an instance of
Mage_Core_Model_Website, null given,
called in
YOUR_PATH\app\code\core\Mage\Core\Model\App.php
on line 555 and defined in
YOUR_PATH\app\code\core\Mage\Core\Model\Store.php
on line 285
Recoverable Error: Argument 1 passed
to
Mage_Core_Model_Store_Group::setWebsite()
must be an instance of
Mage_Core_Model_Website, null given,
called in
YOUR_PATH\app\code\core\Mage\Core\Model\App.php
on line 575 and defined in
YOUR_PATH\app\code\core\Mage\Core\Model\Store\Group.php
on line 227
Actually, I had this error before. But, error display message like Error: 404 Not Found was new to me.
The reason for this error is that store_id and website_id for admin should be set to 0 (zero). But, when you import database to new server, somehow these values are not set to 0.
Open PhpMyAdmin and run the following query in your database:-
SET FOREIGN_KEY_CHECKS=0;
UPDATE `core_store` SET store_id = 0 WHERE code='admin';
UPDATE `core_store_group` SET group_id = 0 WHERE name='Default';
UPDATE `core_website` SET website_id = 0 WHERE code='admin';
UPDATE `customer_group` SET customer_group_id = 0 WHERE customer_group_code='NOT LOGGED IN';
SET FOREIGN_KEY_CHECKS=1;
I have written about this problem and solution over here:-
Magento: Solution to "Error: 404 Not Found" in Admin Login Page
