If the wedding invitations or save-the-dates have already been sent, you do need to send an announcement as soon as possible. You can do this either by mail or by phone, whichever you prefer. Be brief and to the point. No explanation is warranted. Although mailing a card might be more time consuming and expensive, it will save you the awkwardness of any questions that people may naturally ask. The easiest method is to simply have a printed single card made up that states that the wedding has been cancelled.