I’m not sure I can provide any more specific troubleshooting information based on the error message. From what I can tell, Hyper-V cannot start an existing virtual machine. This means that there’s an issue with either the VM’s configuration or with one or more of the existing files. Assuming you don’t have any saved state files or more complex configurations (differencing disks, for example), I recommend trying to create a new VM and attaching the existing VHD to that VM.
I do have another thought: If the problem occurs for all VMs, note that Hyper-V requires your CPU and BIOS to support virtualization extensions (Intel VT or AMD-V) and to have the No Execute option enabled. If your system doesn’t meet those requirements, you’ll generally be able to install Hyper-V, but you’ll be unable to start VMs. I hope this is helpful. If it doesn’t resolve your problem, I recommend posting an issue on the appropriate Microsoft Technical Communities forum (http://www.microsoft.com/communities/Default.mspx) – perhaps someone else has run into this issue and can help.