Hi George,

Welcome to Nikonians!
First make sure that the locking switch on the card is unlocked.
If that doesn't work, the card is likely corrupted.

Using Image Recovery Software like Rescue Pro from Sandisk should be able to recover the images on you card as long as you don't overwrite them.

I have never had a problem with Sandisk or Hoodman memory cards.
Always format the cards in the camera before using them for the first time and after you download the images to your computer.
Good Luck and Enjoy your Nikons!

Best Regards,
Marty