Here is the short list of most common problems that may cause your inability for logging onto the admin panel.
1. Problem: Admin pages won’t show at all, or show without styling.
Solution: You should backup the /admin/.htaccess file (try renaming it to htaccess_bak) and then check whether the site behavior has changed. If it is so, then you need to co-operate with your hosting to find the correct way to use the supplied .htaccess security guard on your website.
2. Problem: Your site is not configured properly to handle PHP “sessions”.
Solution: This issue is a lot more hard to be checked. First, clear your browser’s cache and all the cookies. Then to be perfectly sure, double-check that you do NOT use any Firewall Software Applications running on your local machine as they may have been configured to “disable session cookies” or block their usage. Also, make sure your browser IS ALLOWED to accept cookies, and specially from your own domain. You should check your server’s error log so as to clarify if any errors are happen at the same time that you fail to login.
3. Problem: The most common problem is not having the HTTPS address right in the /includes/configure.php and /admin/includes/configure.php