I have just finished creating my first model on blender and i exported it to unity with fbx and this is what happened

You can see it perfectly fine if you are standing outside the house but if you walk inside the house there is nothing no walls just the shadow.
And you can walk through the model
I am really new and this is my first time i wanted to make my own models for my game so if anyone can help me please.

In Unity planes are visible only from one face, I don’t know how to workaround it with blender, but I hope that the fact helped!