by Julieanne Kost
If files and/or folders have been moved using the operating system (instead of from within Lightroom), Lightroom will lose the link to the file and display a warning icon (!) next to the missing image in the Grid view (as well as in the Histogram panel) or next to missing folder in the Folder panel.
Clicking on the “!” icon will allow you to locate and re-link any files (Lightroom will prompt you to find the files that you moved. In the subsequent dialog, be sure to check the option to Find nearby missing photos if more than one image from the same folder is missing).
You can re-link folders that have been moved or renamed by Control (Mac) / right mouse -clicking on the folder name in the Folders panel and selecting Find Missing Folder.
To view all missing files in your library, you can also choose Library > Find All Missing Photos and then relink them.