Actually, the first thing I would try would be a search for the file, make
sure you search everywere including hidden and system files. When you open a
spreadsheet from e-mail, It puts the spreadsheet in your "temporary internet
files" folder, These items routenely get removed with no chance of recovery.
In the future, either save the spreadsheet first by download>save instead of
download>open or as soon as you change the spreadsheet click "Save As"
instead of just the floppy Icon, when you "Save As" make sure you are in the
"My Documents" folder (or whichever folder you use for your documents)