We recommended you backup your certificate and its corresponding private key in the event that there is a hardware failure or other disaster affecting your Server, the following instructions will show you how. These instructions will also help you export your certificate from one web server and import the certificate and its private key to another web server.
Plesk stores the SSL related files in a httpd.pem file inside a "cert" folder. Both the RSA private key and Certificate are kept inside this file the path to which is:
Backup the the RSA private key from this folder.