What does 就 mean in 你怎么就睡在这里了呀？
The sentence still makes sense, even if you remove 就.


  你怎么睡在这里了呀？


Therefore, 就 is an adverb here, and emphasizes the verb, "sleep".

A few more examples are from 现代汉语词典:


  表示加强肯定
  
  
    我就知道他会来的，今天他果然来了
    我就不信我学不会
    那就是他的家
    幼儿园就在这个胡同里。
  
  
  现代汉语词典, page 697