First make sure you have copied the pictures from your card to your PC. Now make sure again.

When you are really sure you have all of the pictures backed up, look for a button like 'menu' on the back of your camera. Follow the menus on your screen and look for something like 'format card'. Do that and your card should be empty and good to go.

Not sure what happened, sometimes this happens if you format the card in another camera or on your PC. But anyhow that should get your card going.

To make sure your card isn't going flakey, once in a while review the older pictures to make sure they are there. If, on the off chance it can't show the pictures or read the card, get rid of the card and get a new one (this web site: http://www.salescircular.com/ is good for checking out local sales in stores wherever you are).

Good luck!