Well, misconfigured plugin or theme can be reason for that, login to your FTP server and edit wp-config.php and add this line to it.
define(‘WP_DEBUG’, true);
After u add this line, tell your client to enter the address in the browser to and ask him, what is the error message. Check this link for clarification. https://techie-buzz.com/how-to/how-to-fix-blank-wordpress-page-error.html
regards
