Sorry about that, Hilary; this did happen to me, too, when we were first testing out the forum.
Obviously you have now lost that posting, and we can't retrieve it. But if this happens again, an important point is that unless you come out of the forum programme, your posting will still be there (if it's not on your screen, you'll be able to find it by using the backwards/forwards arrows on top left of the forum page). So you can then readjust it and resend it.
If the system refuses to accept it and tells you that you have already sent it, a quick fix is to change the title of your posting slightly.