There are data recovery programs that Might help. But since you've already been using the memory card again. If and how much you can get back is the real question. I'm afraid affroman may be correct. I don't know of specific data retrieval software for memory cards. (recuva is good for hard drives) But Google is your friend. Search the terms data recovery for memory cards and see what you find.
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.