If the login screen just refreshes and reloads the login screen:
Is your sessions.save_path directory writable?
You can use phpinfo(); to see the system specs:
php.net/manual/en/function.phpinfo.php
The sessions.save_path is needed for storing the authentification sessions for the back-end. The front-end works with cookies.
Contact your host to make the sessions.save_path directory writable or
at Apache webservers it's possible to override the default session.save_path from php.ini, by using .htaccess.