How to retrieve SSH access to the server if root password was lost and the server owner is not known?
Only server owner can reset the password for the server and grant it. Locate who is the server owner using any public WHOIS tool for searching hosting providers, for example:
Enter Plesk IP address or a Plesk hostname (it is the URL that Plesk is accessible at, for example, if Plesk is accessible at https://example.com:8443, then example.com is Plesk hostname) and check who is hosting this IP. The hoster would be the server owner. Contact server owner for assistance.