Don't take any more photo's as that will just make things worse.
The pics are still there just the directory entry has been corrupted.
This can easily happen while veiwing photo's on the PC and then just unplugging the camera without stopping it or when the batteries in the camera have gone weak.
There are many photo recovery software out there some are free while others are try before you buy. Just do a search for "freeware photo recovery" and "download".
Here is a small sample of varies software.
http://www.snapfiles.com/get/mjmphotorecovery.html
http://www.z-a-recovery.com/download.htm
http://www.cardrecovery.com/download.asp
http://www.shareup.com/Asoftech_Photo_Recovery-download-51099.html
Never had any need to use any of the software so no idea which are better.
Don't forget to run a virus check before running any of the software.
Just in case your interested here is a universal USB memory card reader that you can get that way you don't need to use the camera for viewing photo's and downloading.
http://www.officeworks.com.au/retail/products/Technology/Digital-Cameras-and-Memory-Cards/Card-Readers/SH11IN1RDR