When the computer says this, it usually means something has occurred to corrupt the card, OR if you are using an adapter (such as one of the ones that holds a micro-SD to make it the size of an SD card) is broken. If you're using such an adapter, get another to try, or try it in another computer. If not, try another computer. Also, if your camera can still read the card, but not the computer, try hooking up the computer cable from the camera directly to the computer and transfer the photos that way.

Once you've transferred the photos off, have the computer go ahead and format the card (or use the camera to format it) and then see if it can read it. Some cameras are fine with computer-formatted cards (check your manual) and others, such as the higher-end Nikons, are not, and require camera-formatted cards.

If the card continues to give you problems though, I would replace it. It's difficult to damage flash memory, but not impossible, and cards on sale are cheap. Check salescircular.com for local sales.