Missing files can occur when you are not connected to an external source that contains your files or if move your files to another directory. If Lightroom can't locate a file, you get a warning question mark icon (circled) in the thumbnail frame.
What should you do? Start by clicking on the question mark. This will bring up this screen. Select Locate and navigate to your files in the directory. Use the directories search function to help. If you are missing multiple files from one location, all you need to do is locate one of the files: Lightroom will automatically link to the others.
You can also see at a glance all the files that are missing by looking in the Navigator panel, in the Library pane.
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