I recently purchased a Kingston 8 GB memory card for my Canon 400D camera and it has worked perfectly until yesterday when after taking around 40-50 photos the message "There is a problem with your CF card. Please replace CF card". came up on the screen. Until this message appeared I was able to view all my photos in the normal way but now I cannot view or download them to my computer. Firstly, are these photos recoverable (as they are from a very special birthday party) and secondly can the card's be 'problem' be remidied?
Have you tried taking the card out of your camera and putting it into the card reader in your PC? If this works OK it suggests there is a problem with your camera.
Also, have you tried a different card in your camera?