Is this a problem with one card or all of them? If it's only one, be sure to format (if you can) using the camera, rather than just wiping the card using your computer. Conversely, if you formatted the card with the camera and it isn't working, try using your computer, although this usually isn't the way to go. If you've tried multiple cards, you know the problem is with the camera itself... at which point you should start with support.nikonusa.com or look around photography forums (Nikon specific if possible) like forums.dpreview.com or thephotoforum.com. Good luck.