How did she get to wear the ballgown again?

The fairy godmother made it for one night, then it disappeared. So...yeah, she shouldn't have it anymore.

And while we're here, instead of thinking up some sentimental brain rotting mush, shouldn't they have been more original with this? Like going against what people would think-have Cindy and the prince fall out, split up etc. Even if they got back together with a sappy ending, it would be marginally better then what it is!