You might have move image files in the Finder (Mac OS) or Explorer (Windows), the link between the files and the Lightroom catalog breaks. When a catalog can't find a photo, Lightroom displays a Photo Is Missing icon in image cells in the Grid view and the Filmstrip. The Photo Is Missing icon is an exclamation point () in Lightroom 5 and a question mark () in earlier versions of Lightroom. In the Develop module, Lightroom indicates that the photo could not be found, or is offline or missing.
Please check the below links that might help you:
Thank you so much but for some reason my originals are gone.
You can check the trash or recycle bin of your system, you might have delete them accidentally
Or you can search for the missing files by their name in your system.