I have read this story on the TechNet forums a few times. Something happened in Hyper-V, either a patch, or a temporary burp, or something else happened that caused a VM to be 'lost' from the Hyper-V console.
That might not be the worst thing.
Then a well meaning and informed administrator creates a new VM configuration attaching tot he existing VHD only to discover that a great deal of time on the life of the VM is gone.
Most likely what happened was that your VM had a snapshot(s). If you claim that it didn't - think back, did you ever create one, then delete it? (and, you have not powered off the VM for an extended period of time to allow that deleted snapshot to be merged back into the base vhd..)
First of all the state of your VM may have been lost for good. The reason? - snapshots
I have blogged a bit about snapshots (here and here, and here) and they continue to cause confusion. I am guessing that the confusion comes from a lack of understanding of the underlying system.
The state of the VM is lost because the differencing disk integrity is broken. A differencing disk is a vhd that is chained onto an existing vhd. The first vhd (the base) stops receiving writes and the new vhd (the differencing disk) receives all of the writes to disk.the other item to know is that vhd is block based - therefore the differencing disk depends on the fact that the base disk does not change. If at any time you connect tot he base disk and modify it (even slightly) - the differencing disk chain is broken and there is not a tool (yet) that can put it back together.
Okay, you are now the better informed administrator - you made this mistake once and you tell yourself - never again!
This is a new post..