This error usually occurs when there is a syntax error in the .htaccess file on your webhotel. Errors in .htaccess files may occur if your page has been compromised or you or your CMS have entered invalid commands in it.
Rename if necessary. .htaccess file to confirm it is the one that causes problems, check and correct the file for errors.
The error may also occur if there is a PHP code error on the web site and PHP errors are set to be hidden.