This message usually occurs when photos have been moved from the location recorded in the catalog.
In the Library, right click on the folder and choose Find Missing Folder. Navigate to the location of the folder and click Choose.
Generally, it is a good idea to use Lightroom to move anything in the catalog. If you move folders or files with the system, Lightroom will not know where the files are.
Let this be a warning flag -
rendition failure also occurs when the drive the application is loaded on NO LONGER has room to function. That happens just before the drive is about to crash from being over loaded. In my case it was an overstuffed TEMP file in the APP DATA location.