You connect to the server over SSH as root. You need its IP address (from the server panel) and the root password.
On the first connection the program asks whether you trust the server's key - answer yes. Then enter the password (nothing is shown while you type - that is normal).
No password, or forgotten it? Generate a new one in the server panel - see the root password article.
Locked yourself out of SSH? The console in the server panel works regardless of network and firewall - sign in there and fix the configuration.