Is your SD card empty? Can you read the SD card on something else? A couple of ideas that may help. Can you go to your phone settings and unmount the card for removal and then reinstall? Can you reformat your SD card? It may help.
in other words, you don't know if it's actually empty or not. you need to verify the sd card is still good. it sounds like the card got corrupted.
Simple thing to try:
- turn phone off
- remove back cover
- remove battery
- remove SD card by pushing on it slightly to release, then remove
- reinsert SD card
- reinstall battery and back cover
- turn on phone and see what happens