sounds very strange, theres a possibility that youve got a virus and its started eating at stuff in that folder, thus changing the file which then makes it unrecognisable.

hopefully it isnt that, and if it is youd be very unlucky but i remember getting a virus a couple of years back, a worm that eat right through files really quickly and left them corrupt and unreadable by there target programmes.

so its a possibility if your not running a firewall and/or good anti virus software.
Most of the files that had become corrupt were recoverable as norton just extracted the rogue code it knew to look for in them and left the original file how it was before so if that is the case then you might be able to get em back.

if it isnt that then i have no idea whats happened