It's very significant that the final line of the book is "and I play" (111.7). Music is the singular passion Billie Jo is tied to throughout the whole book, as well as the legacy her mother leaves her. We can infer that her resistance to playing after the accident is as much due to Ma's absence as it is the pain in her hands, and when her "place in the world" (28.1) is tied to unimaginable tragedy and trauma, it makes sense that she'd shy away from it, even using her injury as an excuse.
The ending of the book finds us in the Kelby home with Daddy and Louise on the verge of being married, the family's new course falling into place. Billie Jo has the strength to use her hands again—both mentally and physically—and returning to the piano is the perfect way to show that. She's got her "place" back finally, and knows that Ma will live on in their family through it.