Why do some objects not show up in the camera view? This is a picture of the normal front view. As you can see, you can see all of the cliffs, trees etc.

BUT



I found out that when I go into 'Camera View', the cliffs and the backgrounds would disappear. This goes same for the rendered version.

As you can see in this rendered version (I am going to add lights later), the cliff and the backgrounds have become transparent. It wasn't like this before, but it became like this!

I'm new to blender, so I can't find out what's wrong. I've tried to find the answers for this problem on the internet, but alas, I couldn't.

I really need to fix this because if I don't, then some of the scenes can go completely wrong! Such as landscaoe scenes. If I start it again from the very start, then I might not be able to finish this in time. Actually, I CAN'T finish it in time.

The link for the blend file is in the comments, but I'm not sure whether it would work properly or not.
The Blender camera has clipping.  It has a start and end distance.  Basically it limits the rays from the camera that Blender has to render, and that can speed up render time (varies).

So right below the panel in the upper right of your screen (and the default view layout), click on the camera icon, and then under "lens", you can find the clipping start and end.



Go to your camera view (numpad 0 or view -> camera) and increase the "end" value until your entire scene is properly in the camera.  Also, make sure that the camera can currently see the most of the scene (maybe at it's furthest away point) when you do that so that it doesn't clip some things in certain positions.  I recommend increasing the "end" value past that point at least some just to make sure it is good.