From recent files. I Copied the file from a thumb drive
Use File -> Open and open files from the file picker dialog. Recent files list is a list of references to files and if you have deleted or moved a file, its reference in recent list may point to nowhere and you get “file doesn’t exist” (Hover your mouse over the link and see the last known location of the file LibreOffice is aware off).
> I Copied the file from a thumb drive
If you restored from a thumb drive, you may not have copied to its original location and hence link in recent files still points to another file (again: Hover your mouse over the link and see where LibreOffice expects the file to be).