I'm hoping someone can help me with this. I transferred pictures from my camera to my hard drive and then in an attempt to import those same pictures to LR CC, all the pics were grayed out saying "Preview unavailable for this file" and then an error message pops up saying "The files cannot be opened by Lightroom. (321) and it lists the numbers of the files. Picture attached. My guess is that something happened when I was transferring the images to my computer. Unfortunately, I've already deleted images from my card reader and can not transfer them again.
What camera model was used to take the images? What exact version of Lightroom do you have installed? Is this the first time you have experienced this problem? Have you imported and developed images from this camera previously? Have you updated your OS recently?