L.M.
I agree with a nightlight, and have you tried sleeping in her room with her for a night or two? Just to see if it's the room, the bed, or needing comfort? If there's already a light in her room check out the room from her perspective, on the bed, and see if there's some strange shadows cast or something that might be invoking a vivid imagination that is frightening her.
Does she have a favorite animal to sleep with? If not - or even if she does - maybe a special trip to pick out a new one might help give you clues as to what's bothering her. If she picks out something that's obviously protective (dog, lion, etc...) she just might be scared of something.
Good luck!