Sadly she has and she said she has no intention on coming back... She retired a few days after her birthday in May 2013.
Here is a post she made on her blog
仁科百華は今日、この日をもって一切の活動を引退させていただきます。急なお話でごめんなさい。この決意をしたとき真っ先にみんなの顔が浮かんで。きっとまだまだみなさんが応援してくれてきた分のお返しができていないかもしれません。更なる飛躍を願ってくれていた方もいるかもしれません。逆に、早く引退していなくなれと思っている人もいるかもしれません。そして私が活動をし続けることで悲しむ人もいました。自分でもすごく葛藤していて。安易な気持ちでやっていた訳ではありませんし、誇りをもってお仕事をさせていただきました。でも、大切な人を傷つけてまで続けていく決心は私にはありませんでした。
こんな私でごめんなさい。
そして今まで本当にありがとう。
Translation (not the best but you get the idea)
100 Nishina Hana will retire themselves by all activity today on this day. I'm sorry by a sudden talk. When I made up my mind to this, the face of all floats at the very beginning. Surely the gift in return of the thing that all of you supported still more may not be made. There may be the person who prayed for a further leap. On the contrary, there may be the person thinking that I retire myself early and disappear. And there was the person who grieved because I continued being active. Conflict even oneself very much. I didn't live on an easy feeling and did work proudly. But there wasn't the determination that I continued injuring an important person to me. I'm sorry with such me. And thank you very much so far.