Then She Was Gone by Lisa Jewell

THEN She was fifteen, her mother's golden girl. She had her whole life ahead of her. And then, in the blink of an eye, Ellie was gone.  NOW It’s been ten years since Ellie disappeared, but Laurel has never given up hope of finding her daughter.  And then one day a charming and charismatic stranger called Floyd walks into a café and sweeps Laurel off her feet.  Before too long she’s staying the night at this house and being introduced to his nine year old daughter.  Poppy is precocious and pretty - and meeting her completely takes Laurel's breath away.  Because Poppy is the spitting image of Ellie when she was that age. And now all those unanswered questions that have haunted Laurel come flooding back.  What happened to Ellie? Where did she go?  Who still has secrets to hide?