The catalog stores the location of the photos you imported into the catalog, so moving or deleting the photos will cause this problem. It also happens if you work on two different computers and file locations are different between them.
The good thing is that typically, locating one of the files results in all the rest getting located as well. This is due to the fact that LR automatically looks for other missing files as well so pointing at one directory where a file is located would cause it to find the other photos as well.
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