I hit this issue when I upgraded my Hyper-V from Windows Server 2008R2 to Windows Server 2012. Some of the saved VMs were not starting up and Hyper-V failed to even enumerate the saved states of the VMs.
You can work around this issue as follows:
- Right click on the VM node and select Delete Saved State...
- The Incorrect state would be deleted and the VM would be turned off
- Now Right click and select Start.
This fixed the issue on my setup.