Did you do a Low Level Format? (takes two or three minutes to complete depending on the card size). I imagine most DSLRs today have that feature.
If it's low level, then that might be the end of it. If your camera does not have a Low Level format option, then it's possible that the files were just deleted (happens very fast), and can still be recovered. As Octarine said: "Just make sure you don't write to the card anymore", and attempt to recover.
Some places to find recovery software: download.com, tucows.com > most will let you try, to see that it works, but for full recovery you usually need to pay for the license (I tested two or three then bought one). Anyone have any idea if there's freeware that does this?