Website is showing web server default page.
There are both index.html file generated by Plesk, and index.php in the document root of example.com and they are shown in the following order:
# ls /var/www/vhosts/example.com/httpdocs/ | grep index
In the Apache configuration file of this domain, index.html is specified first and therefore has priority. So web server shows index.html to clients.
Connect to a Plesk server via SSH.
Rename or remove index.html:
# mv /var/www/vhosts/example.com/httpdocs/index.html /var/www/vhosts/example.com/httpdocs/index.html.bak