There's a number of different issues that cause the errorless admin login behavior you're seeing, but they all go back to Magento not being able to set or read the session cookie. Magento uses sessions to pass error messages between pages — that's why you don't see an error message. Magento also uses sessions to store the "is logged in" value, so not setting sessions also causes the core error behavior.
Possible causes include
Local computer time vs. server time mismatch, causing instant cookie invalidation. Make sure your server time is correct.
Incorrect permissions on var/session, preventing session files from being saved
Incorrect configuration of database/redis/other session storage, preventing saving of session values
A module is instantiating sessions to early, preventing the correct session names from being set
You're a developer using multiple URLs and have multiple cookie domains
Another developer has somehow modified app\code\core\Mage\Core\Model\Session\Abstract\Varien.php, creating a hard to track down bug
The cookie domain in System -> Configuration -> Web -> Session Cookie Management doesn't match the actual site domain.
You're using the localhost as your server domain, and using a version of webkit that has trouble/bugs setting cookies for localhost in some situations.
The short term fix is to just delete your cookie for the domain. That's often enough to solve the problem. If it persists, figure out which of the above reasons is the reason for your error, and take steps to address it (fix permissions, etc.)
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Enable debugging in your site to see error
1.Open your Magento installation directory. Locate and open index.php file.
And find
if (isset($_SERVER['MAGE_IS_DEVELOPER_MODE'])) {
Mage::setIsDeveloperMode(true);
}
#ini_set('display_errors', 1);
Replace above code form below code:
if (!isset($_SERVER['MAGE_IS_DEVELOPER_MODE'])) {
Mage::setIsDeveloperMode(true);
}
ini_set('display_errors', 1);
6.Save this file and upload to the server. Reload your website page to see errors.
For more detail see here
