Your heroine finds this bottle. What does the message inside say? What does she do about it? Where is she when she finds the bottle? What’s going on in her life that makes finding the message meaningful, how does it become a catalyst for her?
Keep in mind that this story idea doesn’t need to be taken literally. For example, she might find the words to a magic spell, repeat them, and be taken to a different time or place or she might receive a wish or…let your imagination take off with this idea.