There can be multiple reasons why your server suddenly stopped responding.
Recent updates in your server operating system.
Recent changes in your server configuration: Network configuration changes. Firewall configuration changes. Port configuration changes in your server's software. Boot loader configuration changes, etc.
System errors caused by a recent server crash.
Server resource over-usage.
Anomalies in the network.
Navigate to your server and use the Virtual Console to connect to your server and perform maintenance, if you are unable to connect to your server remotely.
In addition, Cloud Servers offer the Rescue Mode, which will boot your server using Linux or Windows rescue image and allow you to perform server maintenance.
However, if you are unsuccessful in resolving the problem, please contact Support by submitting a ticket