Photo's taken, not on memory card

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:

http://camerarepair.blogspot.com/2007/12/recovering-those-accidentally-lost.html

You will need to use a flash card reader with this software. If you don't have one, get one, they're cheap:

http://www.amazon.com/s/ref=sr_st?keywords=card+reader&rs=172282&page=1&rh=n%3A172282%2Ck%3Acard+reader&sort=price

The reason this occurred is that you likely never formatted the card in your camera. 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 (press function, select tools menu, select item stating your card's memory capacity, then follow the prompts for low-level format). Keep in mind formatting will also totally erase your card, so again only do this AFTER you've recovered the photos.

Perhaps, you've accidentally deleted them. So, don't worry! Immediately stop using this camera and plunge it to your computer.
In this case, a photo recovery freeware can be the best way for you to go:
http://www.freeware-fix.blogspot.com/2013/02/photo-recovery.html
I believe this data recovery can work perfectly on your camera memory card just in the same way as it completely retrieves all the deleted data of my friend off from his camera.
Note: Next time, you should be more careful about your important data and always back it up timely.

The photos should have been deleted accidentally for some unknown reason. you can try to download a digital picture recovery software to get deleted photos back. here's the step by step info helpful
http://www.asoftech.com/articles/how-to-recover-deleted-photos-from-camera.html