Won't download to my computer

After taking many, many pictures with my NIkkon D70 on my Sandisk Extreme III 4 GB at the tulip farm I came home and slid it into my card reader and received a message that there was nothing on the card. I put it back into the camera, viewed the pictures, plugged the camera into the computer and got nothing. I have used this card before, and the readerso I tried another card in the reader and it works fine. What is wrong with the card and why can't I get the pictures on the computer? Thanks for any help, Dawn