Admins can't read users messagesThis means if you can't find it no one else will be able to, and so if you didn't tick the box to "Save a copy in my outbox" I think you're out of luck LucyB.
The one thing you might be able to do is contact the person you sent the message to and hope that they still have a copy they can send back to you.
What you can do to avoid this in the future is change the default behaviour to be to save messages when they are sent and then if you know you won't want to save a message you are writing uncheck it. The settings are
here (should work when you are logged in, if not Profile -> Account Settings -> Modify Profile -> Personal Messaging will get you there).