I saved my model but pieces vanished? So I've made a map for an upcoming video game and finished it and saved, then closed the program to import it in Unity. I noticed some of the faces are facing the wrong way, so I open up blender again, only to find some of my work missing from the map.

I went back through the auto saves, recover last session, checked all my files and I'm really confused as I know I saved it and I'm wondering if anyone else has encountered this issue before.

Basically the missing pieces are the bottom of a cave and part of a bridge structure and again I know I saved it before closing the program and recovery hasn't worked, so I would really like to know if this is a common issue and how to fix it.

Also due to copyright I cannot upload the blender file or share images.
This might be that you have hidden some faces when editing and not realizing it. I can only suggest to go through your Blend.File and go unhide any objects.  See if that works for ya.