If you add, delete, move, or rename image files or folder containing the image files in the Finder (macOS) or Explorer (Windows), the link between the files and the Lightroom Classic catalog breaks. When a catalog can't find a photo, Lightroom Classic displays a Photo Is Missing icon () in image cells in the Grid view and the Filmstrip.
In the Develop module, Lightroom Classic indicates that the photo could not be found, or is offline or missing.
Links between a catalog and its photos can also break if photos are stored on an external drive that is offline. If the drive is offline, turn it on. If the drive letter has changed, change it back to the letter Lightroom Classic expects.
(Optional) In the Locate dialog box, select Find Nearby Missing Photos to have Lightroom Classic search for other missing photos in the folder and reconnect them as well.
If an entire folder of photos is missing — the folder is grayed out in the Folders panel and has a question mark icon () — you can relink all the photos at once by right-clicking (Windows) or Control-clicking (macOS) on the folder and choosing Find Missing Folder from the context menu. See Locate missing folders.