Take no more pics until you've recovered the photos, take the card out of the camera. Here's some freeware photo recovery software
that should be what you're looking for. You will need to use ayour card reader with this software.
The reason this occurred is that you likely never formatted the card. The card will work fine in the beginning but eventually will corrupt. AFTER you've recovered the photos, format the card in your camera to prevent this from happening again (dig out that instruction manual to figure out how). Keep in mind formatting will also totally erase your card, so again only do this after you've recovered the photos.