NEF/Jpeg viewable on playback - preview not on computer HELP!

I have messed up somewhere and cannot transfer my pictures from the SD card to my computer. Lightroom indicates the pictures are on the card but the preview is not available. Pictures taken previously are viewable/transferable. I'm sure I've messed up somewhere is the settings. Can anyone tell me what I did wrong in the set up to cause this? I can read the card so it isn't the card. I can view the pictures in play back on the camera so it isn't the camera. What the heck!

I have no idea why this happened. I found that by taking a picture using the same SD card freed up the photo's I could not copy before. They are now safe in a file on my computer.
Another play day only different subject.

Glad you got it resolved, I was just reading your first post and wondering until I saw your second post. They may be safe on your computer but make sure you back them up on an external HD and/or DVD.

You can try and use Adobe Digital Negative Converter and convert the NEF to DNG and see if you can open them then?

Thank you.

Did it work?

I've read in several places the advice to format the card, with the camera - not the computer, every time it is returned to the camera.

Obviously, this is done after the photo files are transferred from the card to safe storage.

I've formatted my CF cards in the computer AND the camera. Doesn't seem to make a difference. 90% of the time I use the camera to format them.

Use the camera to format the card after it's been in the computer.

Thanks everyone for your tips. What worked for me as I continued to problem solve to test the card in the camera by taking a random shot. Whatever was causing the initial issue disappeared. I have been using that card without a hitch ever since. I have used the following:
1) copy files from disc to computer by opening the file on the disc - delete pics using delete on camera also delete file on disc from my computer
2) Use PSe9 to transfer from disc to organizer - formated disc after transfer using the camera

Thankfully I have not had a repeat experience.

I appreciate the opportunity to plug into a group of experienced photographers - Thanks again.