Greetings!

While this could be caused by many things, the specific scenario you’ve described has very consistently turned out to be this situation when I’ve engaged customers on it:

Someone installed Nginx on the server, but the website uses Apache. When the server is rebooted, Nginx starts first and Apache then fails to start. This causes Nginx to load the default page, and your website is nowhere to be found. Should this be the case, this would fix it on a Debian/Ubuntu system:

systemctl stop nginx
systemctl start apache2

If that fixes it, you can future proof it with:

systemctl disable nginx

Jarland