There could be a few reasons. It could be a bad memory card, first try formatting the card (this would erase any images but if there are none on there you wouldn't need to worry about it). If that doesn't work maybe try another card.
The contacts between the camera and the memory card could be bad or dirty. Take out the card and make sure the area where the metal contacts on the card touch the contacts the camera are clean and free of debris. Also, make sure the card is in properly.
If none of those work then it is most likely a problem with the camera itself, in that case you would probably have to take it to be serviced.