Certificate Doesn't have a Private Key
If your SSL Certificate is installed to your server, but doesn't have a private key associated with it (This is usually caused by installing an SSL Certificate through the MMC Console to a Pending Request that was created elsewhere) You can use the DigiCert Utility to repair your certificate installation and make sure it's installed correctly for use in IIS, Exchange and other programs. This is only possible if the private key is on the server and the server just doesn't have the private key and certificate associated together.
Repair the Certificate by Clicking Install
To repair your certificate installation, run the DigiCert Certificate Utility on your server, and your certificate will display with a yellow warning sign.
Select your certificate, and at the bottom it will say
"This certificate has not been installed correctly. Press the Install button to repair the certificate."
Click the Install button.
You should then receive a message saying that the repair was successful.
Your certificate should then show up without any yellow warning icons.
Repair a Certificate without a Corresponding Private Key
Tutorial on fixing a certificate that doesn't have a matching private key