First take the memory card out of the camera. Pictures that have been deleted, if that is what has happened, are probably still there, but will be overwritten if you take any more pictures on top of them.
Your camera is not able to undelete pictures, but there are programs for doing this on a computer. You can put the memory card into a card reader attached to a computer, and use an undelete program to recover the deleted files. Undelete programs are available for free download from the internet, try the one at THIS LINK
This program can even get back pictures from a card that has been formatted.
If the pictures are very important to you, you might want to seek help from an experienced person.