Time for the veterinarian to put in her two cents! A 15 year old dog with sudden loss of house training could be a number of things. Often it is due to an increase in water consumption due to diseases such as kidney failure, liver failure, diabetes, thyroid disease, Cushings disease, bladder stones, bladder cancer or urinary tract infections. It could also be behavioral. Changes in environment can really confuse an older dog. At the minimum baseline Bloodwork +/- radiographs should be done. Don't get too excited about the cognitive dysfunction supplements (Anipryl), they do not reverse the disease, only help slow the progression. In advanced age it's hard to get a good response.
Good luck!
Good luck!