Sirius fell into the curtain and was never seen again. Members of the Order of the Phoenix held a funeral for him, and Harry Potter also lost his godfather who loved him.
But on the other hand, Sirius thought he was going to face death, but he didn't expect to return to his infancy. This shows that the Department of Magical Things is indeed elusive, and falling behind the curtain is not entirely a tragedy. Prongs, Moony, Lily, I'm back! Oh, and there are those pesky giant bats! (Cp: sbss)