It sometimes happens. You lose the private key associated with a certificate. Just about everything in OCS requires a private key associated with the certificate.
Here is how you get it back should it go missing.
How to recover the private key for a certificate
1) Double click the certificate and click on the details tab 2) Scroll down to Thumbprint and copy theThumb Printr in the data portion to the clipboard 3) Open a command prompt and type the following (including the double qoutes on both ends of the Thumb Print)