If you would have successfully backed up the Private key and the SSL certificate, you will have to install the SSL certificate as same as installing the new one. Please refer the link below.http://www.instantssl.com/ssl-certificate-support/cert_installation/ssl-certificate-index.html
In case, you have lost the private key, please send us the new CSR generated from your new webserver for the correct common name. So that we can reissue it accordingly.
If you have more queries, please get back to us at support[at]comodo.com.