Have you tried this SD card on your computer?
If not, just try it immediately!
If this SD card can be read normally on your computer, you'd better back up all your pictures inside it and format it to see whether it can still be read on your Nikon camera.
But, if this SD card unfortunately cannot be read functionally on your computer as before, you'd find a way to restore all your inaccessible pictures back at firstly.
In this point, I would recommend you to try a SD card target="_blank">http://www.freeware-fix.blogspot.com/2013/02/sd-card-picture-inaccessible.html
This freeware has offered you four target="_blank">http://blog4mark.blogspot.com/2012/12/sd-card-photo-recovery.html

Yes, I had success with this camera recovery software called asoftech photo recovery, you can download the software from link below
Camera card recovery software

You may check out this step by step tutorial that helped me before
Recover deleted photos from camera

Hope this helps.

You should try some photo recovery software to rescue the pictures on your SD card, here are some you may rely on. Both programs can also recover videos and music.

Photo Recovery Software (for Windows)
Mac Photo Recovery Software (for Mac OS X)

Be careful: Before your pictures are recovered, do not attempt to save more files to the memory card in case the original files(your pictures) are overwritten.