A VPJ project file is saved in case of a crash. With Windows Explorer (the file manager) Look for AUTOSAVED.VPJ in this path...
C:\Users\[user name]\AppData\Roaming\NCH Software\VideoPad. Search the PC for other VPJ files.
Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite. From the HOME tab, that choice is available by clicking the chevron to the right of the Save Project button on the toolbar.
Also check out this thread...
https://nch.invisionzone.com/topic/37202-the-file-provided-is-not-a-valid-videopad-project-file/?tab=comments#comment-98578