The virtual box files with extension .vbox contain metadata the virtualbox hypervisor requires to resolve the guest virtual OS' configuration.
If the main .vbox file is corrupted (i.e. reporting that it is empty) then use the backup .vbox-prev file to recover the contents of the original file.
Do this by renaming the empty .vbox files a temporary name (e.g. rename originalVM.vbox to originalVM-empty.vbox).
Then make a copy of the backup file originalVM.vbox-prev, where the copy will have the same name as the original but with the word "copy" appended to it (i.e. originalVM.vbox-prev is renamed to originalVM (copy).vbox-prev).
It is important to retain the original backup .vbox-prev file it should not be altered or itself renamed.
Now go rename the copy of the newly created .vbox-prev file originalVM (copy).vbox-prev to the original name of the empty .vbox file and be mindful to also change it extension from .vbox-prev back to just .vbox.
That is rename originalVM (copy).vbox-prev back to originalVM.vbox. Now that this is done you may add the .vbox file (guest os) back into the VBOX hypervisor.
This will recover the state and snapshot of the "inaccessible" guest VM. Now delete the original empty .vbox file.