RSA Private Key is generated along with the CSR code on the server where your domain name is hosted. There are two possible scenarios: either the certificate was only activated and not installed on the server or you need to reinstall the Certificate.
If your domain name is hosted on an IIS (Microsoft Internet Information Services) environment, you should be able to restore the Private Key following the instructions provided by Comodo (now Sectigo) Certificate Authority that has issued your certificate. If you are using WHM with root access and a generated CSR code there, the RSA Private Key should be fetched during SSL installation. You only need to paste the domain name and use the “fetch” option which will automatically fill in a corresponding box. You can also ask your web hosting provider or server admin for assistance. If you have a Private key but not sure it matches the certificate you received from the Certificate Authority. In case the RSA Key was deleted from the server and there is no way to restore it, the Reissue is the only way out. You will need to have a new pair of CSR code/RSA Key generated. Before installing your reissued certificate make sure that the old one is completely removed from the server.