Current questions and answers regarding the map editor.

Hello! I'm new to the Map Editor and don't understand anything. I have a problem... I decided to create a map. I worked on it every day for a week, and everything was going well. But one day, I decided to open my Map Editor, selected my map, and THIS happened! The ground disappeared. Some buildings disappeared too! Rivers and mountains have been removed... What is this? What causes this? Is there a way to fix it?