There are only two possible options. A corrupt file system on the SD card or a hardware problem (including a simple one, like a temperamental contact). A few simple ideas to throw in:

1. Have you tried a different SD card? Or the same one in another machine?
2. With the back cover closed, what does ls /dev/mmcblk* print?
3. What does lshal -m print when you keep opening and closing the back cover or waving a magnet in front of the sensor?