"Not lost are you, my dear?" said a voice in his ear, making him jump.
I guess it simply means "Are you lost, my dear?", but I'm not sure. On the other hand, the word order looks odd to me. Is it a common sentence?
A bit context: Harry was lost in Knockturn Alley and he was trying to find a way out to Diagon Alley. Then suddenly, he ran into an aged witch, who said that to Harry.