Yes, FanacooL is right. There have been a number of viruses/malware that have done this sort of thing; I believe loveletter messed up image files?

What I do not understand is that I have not heard of anything RECENTLY with this sort of payload, and I would expect an updated AV to find the older ones?

However, as this is a shared folder, it could be just a straight vandal attack, with no replication capability?

If all that has happened is your file size has been set to zero, you should be able to recover the data.

If the data have been randomly overwritten then set to zero, you are "toast"

Good luck