First of all check for a corrupt .htaccess file on your FTP server. Rename the existing one and try to visit your site. If nothing changed the .htaccess is not the reason for the error. In my case the solution was to rename the folder "plugins" in the folder wp-content. Then try to access the website again if you see the text of your website of course without any graphical elements the reason for the error is a corrupted plugin. Now create a new folder "plugins" in the folder wp-content and move one plugin from the old plugins folder to the new created folder. After every movement try to reload the website if it is possible to see the content it wasn't the corrupted plugin. In my case an internal plugin of wordpress "wp_smtp_mail" was the reason for the error. Hope my solution helps! Good luck!