Sometime magento generate a problem with admin login. Not so worry follow me:
go to here: \magento\app\code\core\Mage\Core\Model\Session\Abstract
and open Varien.php file.
Now comment the three lines from like this:
// session cookie params
$cookieParams = array(
‘lifetime’ => $cookie->getLifetime(),
‘path’ => $cookie->getPath(),
/* ‘domain’ => $cookie->getConfigDomain(),
‘secure’ => $cookie->isSecure(),
‘httponly’ => $cookie->getHttponly() */
);
and try to login again to admin.
I hope its work for you.