Maybe Session variables aren't working on your server? If you are putting in the correct information and login isn't working then that is a possible cause.
Try this... create two pages... on the first page put the code:
$_SESSION['testing'] = "works";
then on a second page put the code:
Visit the first page... then visit the second page. If it doesn't have "works" on it, then session variables aren't being stored properly and you should bring that up with your hosting provider. (it is probably a problem with the session_save_path in the php.ini)