Imported models partially disappear

After I created a model in blender and saved as a .blend file I tried to use it in unity but the side of the model closest to me is invisible so I can see right through it to the other side, if I turn it around the invisible side will change back to the closest side. This happened with a gun model I imported, I tested it out with a mug model which worked fine but I imported in a level design and parts of the floor were invisible (I would fall through them) and the same effect happened with some of the walls.(The nearest side was invisible)