I was doing some work on my home network this weekend, and in the process joined my Hyper-V hosts to a new domain. My demo laptop, which had about 20 VMs saved, threw an error when I attempted to start them, stating that Hyper-V could not find a useable certificate.
To resolve this, I launched the Microsoft Management Console, clicked File -> Add/Remove Snap-in… and double-clicked Certificates. In the Certificates snap-in window, where it asks for the account to manage certificates for, choose the Service account radio button and click Next. Select the Local Computer radio button, click Next and select Hyper-V Virtual Machine Management as the Service account to manage, and click Finish and then OK in the Add/Remove Snap-Ins window.
In the Console1 window, expand Certificates – Service (Hyper-V Virtual Machine Management) on Local Computer, then vmms\Personal, and click on the Certificates node. In the results (middle) pane, you will see a certificate(s) for your previous hostname/domain name. Delete these.
Launch Services.msc, and restart any running services that start with Hyper-V. Now, back in the Microsoft Management Console, hitting F5 will refresh the screen and show your new valid certificate. You’re now free to go back to Hyper-V Manager and start your VMs.