She might be afraid. My son slept great in his big bed, then suddenly wanted us to stay with him again.
He was old enough to talk, so I managed to work it out. He had been watching Thomas and the Magic Railroad (the one with Alec Baldwin) and thought Diesel 10 was hiding in his closet. We gave the movie a time-out (for years, actually) and he slept better.
I kept this in mind and when my second child had trouble being in her bed, I went back and reviewed what she had been watching on TV. We cut out Scooby Doo and everything went back to normal.
I made this suggestion to a friend and she did the same -- figured out which TV show was giving the child bad images (Scooby Doo again for a 2 year old) and cutting it out fixed the problem.
For a while, we also used "Monster Spray" (purple Lysol relabeled) every night in the closets and under the bed. That seemed to put her in control and give her some relief.
Finally, look around the room in the dark and see if any of the dolls or decorations give off spooky images. I remember someone who realized that a clown or doll their daughter had gotten as a gift looked REALLY awful in the dark and getting rid of it calmed things down.
Best wishes. I've been there with a husband working all hours -- make sure you get some sleep, too!