Have you ever tried any recovery software?Some software may solve your problem. My friend has used the software below to recover the hidden files of her memory card. http/freeware-recovery.blogspot.com/2012/07/free-delete-format-recovery.html
It is free. let me tell you how to use it.
First, connect your sd card to your computer. Second, download and launch the freeware. Select "Deep Scan Recovery" to scan your sd card. Then you will see the miracle that all the hidden files come out. You may save them in a new sd card. And I suggest you to check the source sd card to see if there is anything wrong with it.