Great answers by the other tutors, so let me add one: maybe she goes in because she's curious about something, in a spirit of adventure. Perhaps the woods look one way when she first ventures in--sunny, charming, open--and the deeper she goes, the more frightening and ominous they appear. You're right that simply saying "she got lost" isn't very unique, but it actually is pretty easy to get lost in woods, if they're of any size and if one goes in far enough: paths all start to look alike, and they twist and wind, so it's easy to lose sense of direction. And when we start to get anxious, even to panic, that makes it even harder to find our way out of a wild place. So giving the reasons why she gets lost could make it seem more unique and interesting.
As the author of this piece, imagine yourself into the shoes of the protagonist. What would draw you into the woods? And once you imagine yourself in the woods, think what would be disorienting or confusing to you: how might you get lost?
Another option is not that she can't get out but that she courageously decides not to get out until she accomplishes some important goal: a rescue, a solution to a serious mystery.
In a film script, there isn't a lot of room for extensive description, but it might help you to write out details of what things would look like, how "you" would feel at various stages in your journey and once you started to realize you couldn't (or refused to) get out. Even if none of it actually ends up in the script, what you've imagined clearly will help you focus on what's important to include.