virtualbox.org

Could anyone help me with, is there any way to recover the files in a Linux OS in VirtualBox that worked previously but one could not start now? Many thanks.

Yes.

Just like a physical disk, you do not need a virtual disk to be bootable to read files from it. Attach the virtual hard disk of the Linux vm to a working vm as an additional drive. That is the simplest way.

On a Windows host, 7-zip (free, BTW) can open Virtualbox .vdi's, interpret Linux filesystems in the .vdi, and extract files to the Windows host.

I just successfully tried this on Ubuntu, Kali, and Puppy guests.