The most likely cause of this problem (and I’m guessing based on the infrmation you provided), is that you have upgraded from an earlier beta of Hyper-V to a newer version, like the latest Release Candidate. Microsoft has made breaking changes to Hyper-V throughout the pre-release process, and it sounds like you might have had a VM that was in a saved state prior to the update. If that’s the case, you’ll likely need to recreate the VM (create a new VM and attach its VHD(s) to the new VM). This is a pain, but it’s the cost of working with betas. For more information on update “gotchas”, see this Microsoft Knowledge Base article: http://support.microsoft.com/default.aspx/kb/950049. The beta process will soon be over, so hopefully we won’t have to worry about these types of issues in the future.
I hope this is helpful. If not, feel free to follow-up.