The most common cause of a 500 Internal Server Error message is a corrupt .htaccess
file. Additional causes can be a conflicting plugin or theme, or even a corrupt WordPress installation.
You will need FTP access on your server. Navigate to the root directory of your WordPress installation, and locate the .htaccess
file. Rename the file to, let us say, something like .htaccess_old
Now reload your website. If it works, congratulations! You can now generate a new .htaccess
file simply by navigating to Setting > Permalinks
in your WP admin panel and re-saving the settings.