"あなたの髪の毛どこいったの(どこに消えたの?" は英語で What happened to your hair!? もしくは Where did your hair go!? と言います。
All を入れても大丈夫です。What happened to all of your hair? もしくは Where did all of your hair go?
第三者に聞くなら、where did all of ...'s hair go? / What happened to all of ...'s hair? になります。