You didnt' really provide enough information to form a good enough answer. Are you literally plugging your camera in, or do you have a card reader made for your camera? What kind of card is it? How does it plug in? USB 1.0, USB 2.0, firewire 400, is it an internal or external card reader etc.
But it is probably either broken, the port your plugging it into is broken, or it doesn't have the necessary drivers. You can try looking up drivers for the card reader on the manufactures website.