Then, while still using that default style on your site, disable your modifications/plugins and see if you still have this problem.
Note: To temporarily disable the plugin system, edit includes/config.php and add this line right under <?php
PHP Code:
define('DISABLE_HOOKS', true);
Also, you say this only started yesterday? Did you ask your host if they did anything on your server yesterday? Did you do anything on your site yesterday? Do you have an .htaccess file in the directory - what is in it?