Missing files on USB keydrive

Hi there,
It seems that I have lost some very valuable documents from my key drive, (note that only some of the files seem to be missing.) At the same time, the available memory on the drive has increased, (I've been following conversations about similar issues, with the difference being that most people seem to report that their memory space is the same, indicating the files are there, but they are just not visible.) in my case, the files are not there, and the available storage has suddenly gone up, meaning what.....?
I've just made it possible to view hidden files, (on the PC I've been using,)however the files still do not show up.)
i've bee using the drive on both PC and Macand it has, in the past, been absolutely fine.
Any suggestions great fully reciebved!
Vic.