A 500 Internal Server Error message on every page are typically an indication of a misconfigured site configuration file — such as a .htaccess or web.config file in your hosting package.
If you've recently made changes to either of these files, it's worth trying reverting to a previous version.
Common errors include attempting to use php_value directives in your .htaccess file (this won't work on our platform as we run PHP as a CGI — use a user.ini file instead), and incorrect cases in web.config directives.
If you're on Windows hosting, don't forget to recycle the application pool after making changes to your site or configuration, and as the first action to take if you're seeing 500 errors — you can do this once logged into the hosting cp at Web Tools > Manage App Pools.
