It could be several types of fungus, but it sounds like crown rot. If you still have the plant, you should segregate it and make sure not to use any kind of scissors or anything to cut with on this plant and then use those same cutting utensils on another plant. That is a sure-fire way of spreading diseases.

Bet it is crown rot . I was at Lowe's talking to a woman looking at the Phals . I picked one up turned around quick , the top fell off .. not whiplash ..

That happened to me as well. My fiance was walking past my desk and the entire top of the plant fell off....he panicked lol! But it was crown rot....couldn't possibly save it. It's a shame :(