Where are these Files stored on your computer?
Does that happen with only Nef Files?
Also check your Monitor Profile is not corrupt. See http://www.lightroomqueen.com/articles-page/how-do-i-change-my-monitor-profile-to-check-wh ether-its-corrupted/
The error messages mean exactly what they say ... the file has been damaged somehow. If you go to ACR in Photoshop, you will most likely see the same file corruption. Other views could be based upon the JPG preview in the file, which is often un-corrupted when the RAW is indeed corrupted.
This is always caused by a hardware malfunction. It could be the camera card, card reader, USB cable/ USB Port, hard disk or memory. You need to isolate the problem by trying different hardware on this and similar photos.
If the files were once good in Lightroom and now are not, the problem is almost certainly due to a faulty disk drive or failing memory. Find diagnostics for those things and run them.