I can't figure out what happened. She seemed to indicate that she couldn't hear herself singing - which I imagine would make it hard to continue. But the initial goof up made it clear that there was a vocal track to lip-sync to in the first song - which seems weird, since Mariah Carey can clearly sing.
I have a feeling that some fairly serious technical problems happened and that this wasn't really Mariah Carey's fault. From an audience perspective, I think we just knee-jerk think "Why doesn't she just keep singing?" but I have a feeling it's harder to do that than it looks in this kind of massive outdoor production.
Also... wtf... all I could think of was how cold she must have been.