Which file formats? Any chance they may have been mis-named?
Also please read these and proceed accordingly:
These are .tiff files made directly from my RAW image files via ACR then opened and worked on in CS5 before being saved as tiffs - some have previously been opened to watermark them for my FB page but now won't - there are 18 out of 183 images so far that won't open in CS5, Illustrator, or Perfect Photosuite 8 whilst NX2 crashes immediately I try to use it to open them. There is nothing different about the way these files have been produced and as I have several thousand more to work on yet [2 weddings] I should like to find out what is going wrong. The only recourse I can see is to go back to my Neffs and start again - needless to say this is worrying - it takes long enough to get the images perfect without having to keep repeating the work.
If the issue happens randomly I guess it might even be hardware-related and the images might truly be compromised. Do their file sizes appear to be as expected? Can you open them with Preview?
Do you work locally or across a network?
In any case I recommend going through the usual trouble-shooting routines and listing your specs.