Your page stops rendering because of a HTTP error 500:
Also, the certificate for your website expired, so you also have to issue a new certificate from the Free SSL Certificates tool and install that so your website will work even when not bypassing the warning.
Just to be clear, you can still use MySQL databases. However, you must use the mysqli or PDO modules to connect to MySQL, because the mysql_ style functions were removed from PHP a long time ago.
Premium hosting lets you choose your own PHP version, which means you can downgrade your site to use PHP 5.6, which will allow you to use mysql_ style functions.
But I still strongly recommend to take some time to update your code. Running old PHP versions is bad for security and performance, and the mysql_ functions are also notably slower and less secure than mysqli and PDO. So while you can run your site on an older PHP version, please only use this as a stop gap measure, not a solution to the problem. The actual solution is to update your code.