If the host is unknown, then it means that the primary domain name server is not registered for the given domain.
Because the domain name server cannot be found, it is not possible to tell the IP address of the given hostname.
If you are getting this when pinging your IP address then you will need to check that you are using the correct nameservers that were given to you by the registrar of your domain name.
If you are using custom nameservers then you may need to check that you have created these correctly. Please see the following article on Creating Custom Nameservers.