To those saying cortana won't come back because she says she can't in the conversation after the nuke, think about this. IF we assume she is metastable (basically a person from what I've read on these forums) she is now capable of telling a lie. maybe she figured if she told him a way to get her back, she knew he'd at least try, and maybe she figured she didn't want him to take that risk. Does that make sense to anyone?