How can word and JPEG files disappear?

I was looking up old work files that I saved and some picture files and could no longer find some of them. I did run many searches. Some folders are now empty. I’m using Windows XP. The utilities are Clean Disk and Defrag and Norton scan and disk optimiser. There is no Check disk as referenced by Norton but clean disk appears to be the same thing as well as their Optimiser utility.
Anyone else experience this?