There are several programms on XDA site to test your SD card... file, system, bad sectors and so..
Try to open the camera, settings, set SD-card as a direct save method and check results..
If the files are still gone, try to reformat the SD under phone's memory settings.
SDcard > format > erase all > accept.
The card can be broken without any signs from Android, it can be fully recognizable with all capacity shown, but still the card can be demaged and erase itself when Android tries to access it.
