I really liked what Elizabeth said about "monster spray" and using the developing imagination to help her feel better. My eldest daughter started being afraid to go to sleep because of "monsters" about this age. She was VERY articulate, so she went into great detail about it. After trying over and over to convince her there weren't monsters by looking under the bed, shining lights in the closet, etc, one night I noticed a peacock feather in the room someone had given her. We had just read a book about fairies, and I spontaneously picked up the feather, and took her by the hand, and walked her out to the garden. I spoke out loud, calling any fairies that wanted to jump on and help keep the monsters out of her room at night. That became the "monster sweeper" and we would just wave it around the room gently (didn't want to harm the fairies!) each night before I turned out the lights. Once in awhile, we would take the sweeper outside again to give fairies a chance to go back to the garden and pick up some more.
It developed into a very rich imagination play world for my daughter and I, which in turn helped over and over as she came up against other issues in her life.
I don't want to minimize what your daughter is going through. It's a very normal thing to become very sensitive to things, sometimes that don't make any sense at all to us, at about this age.
Acknowledge that she is afraid, offer a hug or snuggle to help her calm down and feel safe, explain things as best you can, but then ALSO find creative ways to work around what she's fearing. It will be a great tool for her later, and it's exactly what this developmental stage can be used to help her learn.
Fiora