Problem with the map editor.

Yeah, that's right.
The map got corrupted during one of the saves. There can be various reasons for this.
To restore it, go to the game folder and look for a folder with the same name as your map (use the search function). You need to pack the entire contents of this folder into a zip archive, change the extension to .h5m, and open it with the editor.
To prevent the map from breaking again, try shortening the name of the game folder. When the paths are too long, the editor tends to corrupt maps.
But make sure there are no Cyrillic characters in the name.