why do objects vanish in unity

I'm new to unity and I have already noticed that when I go to create a room or import something from another programme only certain sides are shown, for example I made a floor with two walls but sometimes the walls either dont show up or only show in one angle.

What am I doing wrong?

Surfaces are only visible from one side with most shaders. In order for objects to be visible, the normals must be pointing outward (most 3D apps have an option to show surface normals), and two-sided objects must be modeled with two actual sides. Also most 3D apps have the ability to turn on backface culling, so you can see how models actually look.

If you design your model in Maya then turn off two sided lightning. If one of your faces shows black, select the face and then reverse normals from the menu. That should do the trick. If it still shows weird, check your faces if they are triangles or quads. If not, use the Mesh cleanup tool to let Maya do the work for you or draw some extra edges.