Poor kid. My daughter has a pretty active imagination, even as an adult, and can scare herself with spooky thoughts.
I agree with Molly. Since it isn't working to talk her out of her fear (that often doesn't work), try to draw your daughter into a gentle conversation in which you acknowledge her fear. "Oh, I hear you are afraid that _________ might happen. Hmm, that is a scary thought. What else are you worried about?"
Once she's been heard all the way through, and not had her feelings denied, ridiculed or dismissed (as she might see it), she'll be in a much better situation for dealing with her own fear. You might try brainstorming a list of what she can do this minute, or later this evening, or the next time she notices that she's feeling afraid. Write down every idea, even if it seems impractical or silly to you. Just get her brainstorming.
At the end of the list, make the suggestion that she pay attention to all the times that nothing bad happened, and that you could help her keep a diary of unscary moments. Once you can help her start to notice the "good" moments, they will probably begin to predominate, and the fearful ones may simply fade away.
Then ask which items on the list you/she can start with – which ones are immediately in reach or seem most practical. You might also give her an opportunity to set some goals for herself – like "By Saturday, I'll take a look in the bathroom mirror." Children often find this wonderfully empowering.
Good luck. You may get some more good ideas from a great little book I use with my grandson, called How to Talk So Kids Will Listen, and Listen So Kids Will Talk, by Faber and Mazlish. Emapthy is a watchword in this lovely little resource, and I have found it to be an amazing parenting tool.