What exact versions of LR and the OS you are using.
Lightroom 5.7 and Windows 10, 64 bit.
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Just for a test update LR 5.7 to LR 5.7.1 and test.
Also Copy the images from the camera card to your hard drive then see if they will open with some other RAW image viewer.
I have a suspicion the RAW images are corrupted and LR can't read them. How old is the card you are using? It might be starting to fail.
Your right, the card had become corrupt as I could open it in DPP but, not Lightroom. At least my camera is ok.