Well, you will have to get someone to come down from the ship and let you in through the "port" authorities. But at the ship, they can look up your name/room number and pull up your picture and verify you. Then you can go to the purser's desk and get a new card.
Its best if you don't loose your card, but of course it can happen.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013