Most memory cards "erases" files by replacing the first letter of the filename with an invalid character in the file allocation table; its space is marked as free, and new files will eventually overwrite the space which was used by the deleted files. Do not try and use your card until you attempt to recover images. I have used Sandisk's Rescue Pro in the past and it worked. Here is the link -

http://www.lc-tech.com/pc/sandisk-rescuepro-and-rescuepro-deluxe/

I was also told that Exif Untrasher also works, but have never tried it.

If the card has not be used since the format the images can be recovered