OK... then, I would start off by validating her fear. Letting her know you also think her story and memories are scary; even if you don't know exactly which one is bothering her now. Then, as Bones suggested, hug her. Let her know you're able to help her, but you might need her to tell you how she wants to be helped. Ask directly - what do you want? what do you need? what are you afraid of? Then, wait.
In some situations, we're not even allowed to claim and own our very real fear; we're talked out of it, shamed out of it for a parent's reasons. And Little Penny needs to know that she is smart; if she's afraid there must be a very real reason for it and once you know what it is, you will try to help...
... and it's possible she's even afraid of "help"... it's like my analogy of Twiggy being like a feral cat... it's possible to overcome a feral cat's natural, instinctive fear of humans... with extreme patience, boundaries, and kindnesses. A bowl of milk, kitty crunchies, or water that slowly but surely moves closer to where you are... while you don't approach kitty at all. Let her come to you... you don't know what will scare her into fleeing... but over time, with kind patience and total openness... she will relax with you and open up herself.
Practical things... can you remember anything Little Penny had that was a comfort or security item? A blanket? Baby Doll? Bear? Can you find anything similar around the house now to "give" her? Little Penny may be remembering something from a pre-verbal stage and supplying something like this can help move to a little older stage, where she may still remember but be able to "tell" you what happened. Rocking chairs are also helpful, blankets with wide silky borders, tactile items... all very helpful.