Quite often, websites stop working for various reasons, below you can find most common issues that generate this type of behaviour.
1. Your VPS ran out of memory.
Proabbly the most common issue is that the VPS runs out of memory and new processes fail to start, and the website goes down. You can check the memory usage either from Hypanel or from command line by running the "free -m" command.
2. Apache / MySQL is stopped
Another common issue that leads to offline websites is that the services responsible for its functioning are stopped.
You can check if apache/mysql are running either via "top" or you an use "ps aux |grep service_name" (ie: ps aux |grep mysql).
If the services are stopped, you can start them with "/etc/init.d/service_name start" (ie: /etc/init.d/mysql start ).
If they fail to start, an error should be prompted, then you can investigate further.
Additionally, if no error is provided and the services fail to start, you can check the specific logs for those services.
Depending on the operating system, apache may be named as "apache2" or "httpd", amd MySQL can be named "mysql or "mysqld".
3. Incorrect Apache / MySQL / CMS configuration
This is more of a general issue and the misconfiguration can occur in various places: mysql, apache, wordpress config, .htaccess file etc. Most of the times a misconfiguration prevents the services from starting correctly or cause the website to be offline. If a service fails to start properly, most of the times an error will be provided.
If it's a CMS (Wordpress for example) misconfiguration, the error may not be explicit, so you will need to check your wp-config.php file for details (database name, user name, password) and make sure that they are correct.
4. DNS not yet propagated / Incorrect DNS configuration
If you've just configured a domain and your website is not working, the DNS modifications may not be yet propagated through the internet or the DNS configuration may be incorrect.
You can check if the DNS zone is configured properly here: http://intodns.com/
If the DNS zone is configured correctly, you probably need to wait for DNS modifications to propagate. This may take up to 48h, depending on the internet provider that you have.
In some cases the website may not be available to you (appear offline) but work for others.
If that's the case, you will need to make sure that you can reach to your VPS. This can be done by running a traceroute to the IP address of your VPS.
If there's a routing issue (ie: temporary issues with a local ISP ), the packet flow will stop before reaching your VPS.
If you are able to access the IP of your VPS, you can connect via SSH and check your firewall since there's a good chance that you are being blocked.
If you are unable to access your server via SSH, please open a support ticket.