Photo Gallery

All pictures have disappeared from the gallery.  This is second time has happened.  First time just entered card had in my camera and forgot the older pictures.   Today, happened again.  I have also downloaded a photo site, Picasa, which I really like.  All pictures are gone from this site.   How do I recover them and what is happening to cause them to disappear?  Message I get is "cannot locate photos.  I was working on the pictures to transfer to disc to save.      Thank you.