The Promise by Danielle Steel

Young architect Micheal Hillyard and artist Nancy McAllister are determined to get married despite his wealthy mother’s disapproval. Then minutes before their wedding, a terrifying accident and a cruel deception separate Micheal and Nancy–perhaps forever. Each pursues a new life–Nancy in California, Micheal in New York. But eventually nothing–and no one–can keep them apart as they keep their vow never to say good-bye.