Great forum post on how to resolve the error “cannot connect to the virtual machine because the authentication certificate is expired or invalid”.  It all has to do with a certificate on the Hyper-V server that is only good for one year.  Al you have to do is to stop they Hyper-V Management Service, delete the old cert and restart. Any VM’s you couldn’t connect to, save their state and restart.

Thanks to Mark at http://social.technet.microsoft.com/Forums/en-US/winserverhyperv/thread/1a3e8f1b-379b-4569-9b39-bb568d2a7e3f/ for the answer below:

Here is what you need to do, there is a certificate for the Hyper-V Virtual Machine Management service that has expired. By default the certificate is a self signed certificate and is only good for one year. Here is how to resolve it:
1) Open Services.msc and go to the Hyper-V Virtual Machine Management service and stop the service.
2) Next go to Start - Run and enter MMC - Ok
3) In the MMC go to File - Add/Remove Snap-in, in the list of Available snap-ins select Certificates then Add.
4) In the next window select Service Account and Next. In the Select Computer select the default of Local Computer then Next.
5) Now under the Service Account drill down to the Hyper-V Virtual Machine Management and select it then Finish and OK.
6) Now in the left hand pane expand Certificates, vmms\Personal and highlight Certificates. In the right hand pane double click on the certificate, should show the Issued To as the host machine name.
7) On the General tab of the certificate at the bottom it should show Valid from and a starting and ending date. The problem is that the certificate has expired.
8) Now close the window for the certificate and then in the right hand pane right click and select delete.
9) Go back to the Services.msc and restart the Hyper-V Virtual Machine Management service.
10) Back to the MMC console and refresh the Personal\Certificates and you should see a new one there. Double click on it and verify the new valid dates.
11) To be able to access the VM's now you will either have to restart the VM or simply use the save state then start the VM back up.

Hope this helps.
Mark