It probably means the other flowers are on the way out. Although I had one recently where half the flowers dropped, but the oldest ones hung on another three weeks. Never seen that before.
But just because the flowers may be over does not mean the end for you plant. With care it will flower again for you in several months time.
I was really suprised when my first Phal flowered for the second time. Of course I knew they could, but I had thought it would need really special treatment, rather than just being looked after by little old me. But it did flower again, and again and again, which is what made me decide to get more and get into keeping Orchids even more.