Hi,
There is an option in "Tools" "Options" "Document Handling" called "Create Backup Copy on Save".
Though wont help you now if it wasn't turned on.
If you had recently done a preview then there will be a PDF with a random name like _62o214ds4a9136gk.pdf in your %temp% directory, just open windows explorer and type "%temp%" in your address bar (without the quotes).
Good luck
Bruce