As long as the toddler is not sleep walking, which is another matter, I would say there is almost always a reason for a change in behaviour. It can be anything, but something triggered this, and then it becomes a pattern, and much harder to break.

There have been some good suggestions here. All I can add is that it is really important not to let your toddler stay in bed with you...take her back to her room, don't send her alone and stay with her there if need be until she seems settled.

I think you need to reward her for staying in her bed, rather than punish her by denying her a bedtime story.