I had that kind of problem with a Sandisk last year. For some reason, initially the card was ok but after inserting it in the camera and shooting few test shots, I decided to format it.
It stopped working. And the camera (a Canon 350 at that time) froze. What a panic!!!
I got it back to the store I bough it and they replaced it. So I guess that your problem is with that card. Try to read it on another device.
Claude