I’ve seen this happen a couple of times now.   A working Hyper-V server, when restarted, “looses” virtual machines!   No errors, and no trace of the VM! Scary!

Turns out, it’s because the anti-virus software is scanning the virtual machines as they’re loaded and gets in the way.

Here’s a good article describing the problem and which folders you need to exclude from the AV scan:

http://social.technet.microsoft.com/Forums/en-US/winserverhyperv/thread/e81d0f59-18b5-4214-94ea-6cde883fdd4e