8GB ultra memory card "unable to read"

I photograhed about 150 photos on my Sony A33 DLSR.  All my photos were on my camera to review.  Seems everytime I review, I loose more photos “unable to read”.  I know only have 32 photos.  When connected to my computer, the “unable to read” photos are even recognized, only the 32 that is left.  Any suggestions?  I REALLY need these photos.