I hate when the expectation is for long term care, as this client seemed to indicate to you, abruptly comes to a halt. You truly feel blindsided. I have had a couple of these and it does hurt as the child was a great fit and of course the income seemed reliable. I now don't take heed when they tell me they are in till child goes to J.K. because I know so many things can change that would alter that long term prognosis. Fortunately, they are leaving for a reason that does not have anything to do with the care you provided. Ask them to please refer you to their friends and end this professional relationship in a positive, upbeat manner. This client will remember this as she probably does feel bad about leaving your daycare.
Feel sad for a bit, and then get busy filling that spot I wish you lots of luck in your search.