my map doesn't load in full

Hi, let me start by saying that I am just learning this tool. Yesterday I was playing around with creating a map (scrn
). I’m sure I saved everything correctly and turned off the program. Today I wanted to continue learning and only so much of the map loaded (scrn). Outliner shows the other parts of the map, but when I click on them it only shows me a transparent lump where the map should be. Does anyone have an idea what I am doing wrong?