It can't find where to boot...
Try with Gparted. Download the lastest iso of it, and attach it to the CD of that VM, and make it boot to it.
Select the drive that should be bootable, and add the flag "boot" to it, and apply the changes. That should change that drive to be bootable. After which, run Converter "configure machine" against this system. After that, try to boot it again.