First off I don't own a 313, only a 323 - so take my comments for what they are worth.
How do you have your NAS configured? (RAID0, RAID1, JBOD, Seperate Volumes,....)?
Do you have any fun_plug's (or third party plugin's installed)?
What did you do just before the files became inaccessible?
Do you have a backup of your data? (This really should be question #1)?
What is your network setup?
Can't offer anything else until we know some of these answers.
Regards,
Wiggs