A lot of things could have occured that caused a wedding to be cancelled, so you need to be sure what caused the cancelation before offering your response. Be repectful and courteous when asking why the wedding is canceled. If it was just a simple cause like the venue being full, offer your help in finding a new place. But, if there is something troubling happening between the couple try and help them figure out a solution or at least let them know you will be there to help them.