Greetings!
I’m sorry about the trouble this has caused for you. I cannot give a precise answer to this question, but I can give a generic one. What it most likely means when this happens is that the services that run your website were not configured to start when your server boots. This means that you need to log in via SSH / console and start those services.
Let’s say, for example, that the Apache web server runs your website. First, I’m going to start the web server like this:
Ubuntu / Debian:
systemctl start apache2
CentOS / Fedora:
systemctl start httpd
Next, I’m going to configure the service to start on boot so that this does not happen again:
Ubuntu / Debian:
systemctl enable apache2
CentOS / Fedora:
systemctl enable httpd
The important thing here is that I do not know what applications are required to run your website. However, this is something that you should be aware of from it’s initial configuration.
Jarland