Hi,
Have you tried playacting with her about what she might be afraid of? My SIL was TERRIFIED of a clown doll that had been in her room since she was a baby, and it took her years to be able to realize that she didn't have to sleep with it in there. Maybe if your daughter could pretend to be putting a favorite stuffed animal to bed in the room by itself (in her place) and then she could play the mom, maybe what she tells the toy would give you some ideas about what might be frightening her? Worth a shot, maybe?
My five year old recently decided she was afraid of the dark--I am fairly certain it's the shadows the streetlights make on the items in her room that are bothering her, but also may be the sound of the heater coming on. She received a really great, white teddy bear for Christmas--I believe it's called a "Glow-E" (I've seen them at walmart)--which turns on when you press his paw and cycles through really beautiful, bright colors continuously. She's rarely has any problems now with this bear (though she still makes us keep the door open a teeny tiny crack--we close it once she's asleep).
We also play music softly for the kids at night when they go to bed, which seems to distract them and helps in getting them calmed down and sleeping faster (and therefore less likely to lay awake dwelling on scary things). It also covers up any possibly scary noises.
Hope this helps!