I am doing the lerpz escapes tutorial and just made the Main and the Game Over Menu. Now I reopened the Game Scene and my level geometry is gone.
I can only see the objects, if I select them in the hierarchy and then I see only the blue outlines. What happened? I cannot place the Laser Traps if I dont see the level geometry. 
Read the documentation and familiarize yourself before asking a question like this. Instead of rushing into development with a tool youâre not proficient with, taking the few weeks to create a basis for yourself and learn shortcut keys and what all the buttons do is ultimately going to be more helpful than asking a question every time you get stuck.
View Modes The Scene View control bar
lets you see the scene in various view
modes - Textured, Wireframe, RGB,
Overdraw, and many others. It will
also enable you to see (and hear)
in-game lighting, game elements, and
sound in the Scene View.Draw Mode
The first drop-down is for choosing a
Draw Mode. You can choose to view the
entire scene Textured, Wireframe, Tex
- Wire, Render Paths, or Lightmap Resolution. This has no effect on your
game when it is published.Draw Mode drop-down
Textured shows surfaces with their
textures on, like in the game.
Wireframe shows the wireframes of the
meshes in the scene. Tex - Wire shows
wireframes on top of the textures.
http://unity3d.com/support/documentation/Manual/View%20Modes.html
Of course:
I thought it could be a problem with my windows service pack because Unity said something like that in the console log. But after installing the service pack the problem still exists.
I got it. Somehow the âdefaultâ layer was deactivatedâŚ
