What can I do to prevent my terrain and camera from disappearing from the hierarchy and project interfaces?

When I first downloaded the free Unity program I use it to create a small from a textbook I bought to learn about the the game engine. Soon I was finish with my terrain and save it for the next day. However, when I start up my project the terrain was gone. I don’t know how to get it back, so what can I do to prevent that problem from ever happen?

Save the scene, then load the scene when you next start Unity. Note that there is a difference between loading a project and loading a scene.