A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
Where are files stored while you are editing them? The most common causes of this problem are editing files while on an unsupported cloud service like Google Drive or iCloud, or editing files while they are on a USB/Flash drive.
If the file that is 808MB doesn't display the pictures, then you'll probably have to replace them. When you do that, try using Insert>Pictures>This Device and Insert>Video>This Device commands. Use File>Save As as you go, changing the file name slightly each time. Then you always have something to go back to if there is a problem.