This happened to me a couple weeks back with my 64GB card, which was pretty scary. What worked for me was to turn off the system, take out the memory card, turn it back on and once you get to the home screen reinsert the card. This will prompt you to restart the system and force it to rebuild...
Mr.AahH!! is free today too: http://itunes.apple.com/us/app/mr-aahh/id326819173?mt=8
It's a fun little game where you swing from platform to platform and try to land in the center of each - I recommend it! :)