I use unity 5.0.1p4.
Look at this screen shot. All UI elements are invisible on scene view. (wireframe visible only)
[46169-스크린샷-2015-05-11-오후-23339.png|46169]
But In game view, UI element displayed well.
What should I do?
I know select layers menu in the top right corner of unity. However, I set UI layer visible. It doesn’t work. 
Try to change the scale of the ui element, sometimes it goes to 0 for no reason.
This can also happen if the UI component is not a child of a Canvas.
Try use Assets>>Reimport all
it is usefull tool and solve many problems of bugs
I had this problem, none of the above resolved the issue.
In my case, the issue was that Post Processing was selected in the scene view layers.
Unticking “Post Processings” worked for me.