A virtual machine has be shut down in some way, and upon rebooting all the changes since data/time X have been lost. After viewing the file system you find that the VHD file’s last modified date is X time in the past, you also find that under the Snapshots directory that a .avhd file exists with the current date/time, holding all your changes to the original Virtual Machine file.

The only way to restore the Virtual Machine to the current modified state is to manually merge the Snapshot Differencing Disk (AVHD) file with the main hard disk file (VHD) and then create a new Virtual Machine using the existing, merged VHD file.

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