Models in Unity Appear invisible?

No matter how i import my models into unity, the models still appear invisible (invisible as theres a clear box with the x,y,z arrows where the model should be) i first thought it may have been my graphics card but now the models that used to appear in the editor are invisible also , so it knocks out the flipping normal theory, i even reinstalling the engine but still have the same problem . (the model appears in the preview perfectly but not in the editor) i even tried scaling the model to huge sizes but no luck..any suggestions please.. (im using wings 3d and sketchup with playuptools)

Do you drag the prefab created by the model directly into the scene? Have you tried just dragging the mesh and not the prefab into the scene? Although, you might try scaling more. I know that sketchup makes the models very big and if imported into unity it is possible you are in the middle of the object looking at the insides of the planes that make it up. You should try to flip the normals though, then you will know if you are in the inside of a gigantic object.

I've had this problem before; my solution was to make sure I could see the default layer. Some how this got unchecked for me and I had to turn it back on.

At the top right there's a drop down box ("Layer"), make sure Default is on.