First off, don't write anything to the card until you've completed the recovery.
You'll need a card reader and some software - I recommend Piriform's Recuva. It's free and it works well.
Install Recuva onto your PC, put the card in the card reader and run the software. Make sure you tell Recuva to restore all files it recovers to the PC and NOT the card.
Once you've got them all back you can use the card again.