What happens when you visit /wp-login.php ? Is the .htaccess itself set with proper permissions? My wp-content, wp-admin, and wp-includes all have rwxr-xr-x / 0755 and my wp-login.php, .htaccess all are rw-r--r-- / 0644
Is this a fresh install? You can also check the server logs and the last resort, but perhaps the easiest, contact your host and have them take a look at root permissions and config.