Me too.
I started to get this error when my PC BSOD'd. I was running F15-F16 upgrade (fedora) and suddenly BSOD because of overheat.. after restart latest snapshot nor current image did not start anymore. I KNEW this would happen some day
EDIT:
Got it working. I recovered a "Fedora15.vbox-prev" -configfile, then got the broken F16 upgrade to start, it gace some kernel panics and did not want to load. Then I used "recover snapshot" and hot back to login prompt..
Only thing i did, was install somekinda filerecovery tool and recovered the only file i found for VM, i tried to search for older images but the file i mentioned earlier did the trick! *confused*