Perhaps it is attacked by virus or thr FAT is broken. You may need a data recovery software to recovery the data and then fix it. I will recommend you a totally free software. It will help you recover the invisible data. Download a free one here: http://freeware-recovery.blogspot.
Connect your card to your pc and launch the freeware. Then click data recovery on the demo and let the software scan the invisible files. After the scanning you will see the lost data in front of you again. Preview the files and decide which to save.
By the way the page I give you also has many information about the freeware so you will find it quite easy.