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I reinstalled Virtualbox and tried to launch the VM again. But it is still the same. I renamed the .xml and .vbox files several times (adding -bad2, -bad3, etc. at the end) and now don't know which files should be the right ones.
I also unchecked the EFI option and then the VM was asking for Windows installation, nothing else (I skipped of course).
I am just losing hope... I need this VM for my job!

Uninstalling and reinstalling Virtualbox leaves the guests and configuration behind for use later.

At the risk of being accused of recommending to lock the barn door after the horse has escaped, Important things, like things you need for your job, should be backed up. Backups are as ubiquitous as telephone and internet and needing to plug in and/or recharge. Restore from a backup.

But, yes, that horse has escaped, right?

Log out of your account and back in again, so no Virtualbox processes are running. Gather all of the W10.vboxes you have and move them to another folder. Now take one, put it in the guest folder, rename it to W10.vbox, and try it. If it works, good. If it does not, Log out of your account and back in again, so no Virtualbox processes are running, move the failing W10.vbox to another folder, then move another .vbox in, rename it and try again. Rinse and repeat from the log-out, until you get a working guest, if you get one.

The Virtualbox VMs are located in a part of my disk that was not backed up. I had never realized that I had something important to back up there... I kept all of my documents but I may have lost the software and a part of my work that was saved in the software's folders. So yes, that horse has escaped.

I tried all the .vbox files but none of them works. There is something a bit strange : the two older .vbox files appear being 0 octet (and the VM can't be launched at all) while the other ones are 8.1 ko and you can at least try to launch them (but they get stuck).

Is there a way to fix one of these vbox files?

Next to the .vbox file, there are also a .nvram and a .vdi. Is there anything to try with these?

Or, if not possible to fix the guest, would it be possible to "extract" a file saved in the W10 guest's C: from the a .vbox file?

A .vbox file does not contain any data from the OS inside of the vm.
All data from the guest os is stored in a virtual disk, called foobar.vdi or whatever format you have choosen at creation of your vm.

If your guest had no snapshots, it could be possible to make a new Windows 10 guest with the settings as much like the old guest as you can remember, and replace the new guest's virtual disk with the old guest's virtual disk. If the data on the virtual disk is still good it might boot. If it does then you're 7/8 of the way there.

towo2099 wrote:A .vbox file does not contain any data from the OS inside of the vm.
All data from the guest os is stored in a virtual disk, called foobar.vdi or whatever format you have choosen at creation of your vm.

Hello towo2099. Thank you for your help and this explanation. Is it possible to browse a .vdi file and extract files from it? If yes, how to proceed please?

on a Windows host PC I can open a .vdi with a standard non-encrypted file system on it using 7-zip, then extract files. Linux probably has a compressed file manager that can do the same thing.

Or you could boot the dead guest with a Linux Live-CD that can read Windows NTFS, then email the file to yourself from the Live-CD's web browser.

Or install Windows in a new guest and then attach the dead guest's drive as a secondary drive. You could install Guest Additions in this new guest then use a Guest Additions Shared Folder to transfer files out onto the host.

Hi,
Considering that my Windows VM was lost, I created a new one, thus creating a W10_2.vdi. I switched it off and then attached my old W10.vdi as an additional virtual disk to this new guest. Then I started the W10_2 machine again and, surprise , it booted up on the W10 drive (the old one) and it is perfectly functional... Of course I hastened to backup the files that I wanted to get back!
Does anyone understand what happened and could explain it to me please ? So far I did not dare switch the guest off.

Hi,
Here are some news, for your information. The old W10 guest still doesn't start and the new W10_2 still work, booting up on the old W10.vdi (although I don't understand why on this drive and not on the W10_2.vdi drive). However so far my problem has a solution and I even didn't have to install all the software again.

Glad you're up and running! Now that the horse has returned, take measures to see that it doesn't escape again...

The best backup of a Virtualbox guest is a full file & folder copy of a completely shut-down (not save-stated) guest folder, containing all the files therein, as well as any disk files that may reside outside the folder. The backup can be FC file-compared, or hashes can be taken and compared for off-site backup where FC would not be practical. If the disk file(s) are inside the folder right next to the guest's .vbox file, then that backup can be restored to any capable Virtualbox host running any supported host OS. If a disk file is outside the guest folder, then the .vbox file contains an absolute path to the disk file, and that path must be re-created on the new host (or the .vbox file needs to be manually edited). Virtualbox 6.0 and later allow moving a disk file into the guest folder next to the .vbox file, using the main Virtualbox window's File menu, Virtual Media Manager, Move command.