What Once Was Mine by Liz Braswell

"Desperate to save the life of their queen and her unborn child, the good citizens of the kingdom comb the land for the all-healing Sundrop flower to cure her . . . but someone mistakenly picks the blossom of the Moondrop instead. This shimmering flower heals the queen and she delivers a healthy baby girl with hair as silver and gray as the moon. But with her mysterious hair comes dangerous magical powers: the power to hurt, not heal. For the safety of the kingdom, Rapunzel is locked away in a tower and put under the care of the powerful goodwife, Mother Gothel. For eighteen years Rapunzel stays imprisoned in her tower, knowing she must protect everyone from her magical hair. When she finally decides to leave the only home she's ever known--to see the floating lights that appear on her birthday--she gets caught up in an unexpected adventure with two thieves: a would-be outlaw named Gina, and Flynn Rider, a rogue on the run. Before she can reach her happy ending, Rapunzel learns that there is far more to her story, and her magical hair, and her future than she ever knew."--Publisher's description.