Chances are, SSH/SFTP Access is disabled on your server. You can enable it using the toggle switch located in Server Settings under the Credentials tab.
Your server also comes with Fail2Ban, an intrusion prevention software framework that protects it from brute-force attacks. If you attempt to connect multiple times via SSH/SFTP using the wrong password, it will lock you out of your own server.
To regain access to your server, you can try connecting from another IP address, or open a support ticket so we can remove your IP address from the system.