Tridion Docs - The certificate's private key could not be accessed.

The .NET web error page mentions the following error or you see the following error in Windows event viewer:
The certificate's private key could not be accessed. Ensure the access control list (ACL) on the certificate's private key grants access to the application pool user.

Open start – run – mmc.
In mmc, do File – add remove snap-in. Select certificate – Computer Account – Local Computer – Finish and OK.
Navigate to Certificates – Personal – Certificates – right click on the certificate that you are using and choose All Tasks – Manage Private Keys.
Give the IIS pool identity read rights to the private key of the certificate.
 

Also see:
Troubleshooting Relying parties and Certificate issues  
Error: 'ID1039: The certificate's private key could not be accessed