Daphne, first of all, just want to make sure. When you say on the memory card, do you mean an external (removable) SD card, or the internal memory of the phone itself? Just asking because sometimes people confuse the terms and say one when they meant the other.
What kind of phone do you have?
There are programs that can find deleted files, but some are difficult for the average user to understand. I'll let someone else advise you what to use. But I did learn today on another of our forums that when this situation occurs, that it's important to stop using whatever the deleted file was stored on, because if you do something that overwrites the little section where it was stored, it may not be possible to recover. So definitely don't take any more photos with the phone until someone advises you further.