It may only exist as a thumbnail if the actual photo file has been overwritten. Thumbnails are separate and are held in a 'cache' by Windows. That's all you're seeing and that's all it is. The cache has evidently not updated itself since the picture file was overwritten, which is why there is now a mis-match between the thumbnail and the existing filename.
My digicam doesn't duplicate previously used image numbers even if the card is formatted (the camera remembers which index numbers have already been used) so perhaps your camera doesn't have this feature or you've reset the camera's image counter somehow. Even then, an overwrite process should trigger a Windows warning to which you have to give your permission -- so it's very odd unless you weren't taking notice and allowed it.
Windows XP keeps a 'database' of the thumbnails in each folder, it's called 'thumbs'db' and it's hidden. If you unhide it you can delete it. next time you open that folder a new database is created which should then match the actual images it represents. I imagine same applies to Vista (which I don't have). That won't bring your overwritten picture back but it will fix the mismatch.
Edited by pip22, 29 April 2008 - 04:04 AM.