As dextermat has said, Recuva is the recommended program for this.
However, if you've been using your memory card since they were lost, they may not be recoverable.
What happens when you delete something, is it is simply removed from view. It isn't actually "erased." As you write new things to the storage device, they are written over top of the previously "deleted" files.
There's a guide on how to recover them
found here. You may or may not be able to recover all of them. It depends how much you've used the memory card.