One free program is RECUVA
There a hundreds of similar programs that cost you.
But you must stop using that card immediately, otherwise you might write over any hidden images on that card.
Use RECUVA to recover any images that you can.
After you do, before you use that card again, you must format the card with the camera.
That will (in effect) delete everything on the card.
Things to think about:
When installing a new memory card, Always format the card with the camera.
Some cameras have internal memory in addition to the memory card.
If you have no card in the camera, the pictures you take will go to internal memory.
To read internal memory, remove the memory card.
Do not put someone else's memory card in your camera.
Do not put your memory card in someone else's camera.