Photos on Card, but PC says that no files are present

I can see 3.8GB of pictures on the screen of my Pentax K20D camera, but my computer shows that I have no files on the microSDHC 16GB class 4 memory card. I tried 2 other computers, but all of them said that there were no files on the card, but it indicated that 3.8GB was being used. This is not a new memory card, I have used this card many times before. How can I get my pictures off this memory card?