My phone did the same thing a couple of weeks ago... the way I fixed it was removed the SD card, turned off phone and computer, restarted both, then when my phone was all booted up I put the SD card back in and voila, it worked. I'm not sure exactly which part of this fixed it but nonetheless it did fix it... at least for me.
I think its just one of those weirdo random bugs that pops up from time to time. Hope this helps.