I don't know about Yahoo, but Gmail takes note of the people you get emails from and keeps them on file even if you've deleted the email. So if you remember a bit of the name or address, try a search in your contacts file.
The links from this page might help but I can't check them myself. (I've not got a Yahoo! account. I assume that you need to be logged into Yahoo! to get the links to work):
http://answers.yahoo....20090624095724AALbGyQ
not the computer's recycle bin - does Yahoo email itself have a 'trash' folder or something similar? Emails may go there and wait to be finally deleted (or vanish after a month or something).
Hooray! Thank you all so much. I have located the trash folder ( not understood what it was before) and retrieved the Email. I shall know what to do next time.It is very comforting to know that so many of you are there to help, as I have had several queries and have never been let down.