Disappearing Photos

Hi all - I'm hoping someone can help with a weird problem. I went into My Pictures folder to open some pics of items I'm listing on eBay. As soon as I ran the cursor over the pic, it disappeared, was replaced by the default jpeg icon and says 0 bytes like there's nothing there. None of them can now be opened in Windows, PhotoShop, etc. - every program says there is no file to open. A couple of folders on my desktop with pics open perfectly, so it's just the ones in My Pictures. And I'm starting to panic a bit because I have literally thousands of photos in there. Any ideas before I grab a paper bag and start hyperventilating?! Thanks!

--A