Unity Editor deleted everything in Heirarchy

Bit hard to help when we've got so little to go on.... what action were you doing when this happened? Were you in play mode or editing the scene? What version/OS are you using?

If you think it's a bug in Unity, you're best off reporting it as such rather than posting here.

I just opened it and boom eveythings gone

So you opened a new project? Or a scene within the open project?

Also, try the solutions here: http://answers.unity3d.com/questions/588848/custom-editor-window-wont-open-after-layout-crash.html

Scene in the project. Clicked the unity logo on the desktop. Opened up the scene called "ProtoType" and this happened. This never happened before Unity 4.6