All right, so you can't even get to example.com because of Error 500 correct? In this case you can use an FTP, login to your server and if you see the .htaccess rename it to .htaccess.txt. Now you can try checking if the WordPress is available.
Second option is to rename your current theme directory, so let's say that your current theme is called wp-content/themes/mytheme/ rename it to i.e. mytheme.hold and see if the WordPress is back on. When renaming your current theme, the WordPress will automatically activate your default theme. The default theme is either "Default" prior to 3.0 or "TwentyTen" after that.
Third option are your plugins, follow the second option and do the same for wp-content/plugins