It means that your VM couldn't to connect hard disk device on start by some reasons. In order to try to figure out problem please send us problem report (Help->Report a Problem) and post here it id.
I moved all my hard disk images to another folder and this caused my VMs to not find them. I went to the configure menu, hardware, and browsed to the new location and chose the disk. Now it works again. I also had this happen once when I chose to rename a hard disk. Just had to tell the VM where it was again.