After the obliteration of Roa, Ciel lost her characteristic immortality.
Ciel naturally reverted to having a body that would "die if killed" since the World no longer needed to kept her alive to sustain Order.
However, Ciel was still tough. No matter what, Ciel's body was still the one that "survived" after being killed by Arcueid and abandoned by Roa's soul. Its power of regeneration was exceptional.
Still, she would not resurrect if killed. In order to kill someone who could regenerate like Ciel, it was best to destroy the brain, where the Magic Circuits converge.

talk. - Ciel's Immortality