No way to know what happened, you had the pictures on the SD card, and it worked fine for a year? Or did you just install and SD card and the files are gone? Did you change the storage to the SD card in the photo app and told it to move all files there? Did it? You can try connecting the phone to a computer and see if you can find the files that way, maybe they were moved and the photo app just is not showing them to you. Can also try an undelete utility, Recuva is pretty good, they have some phone recovery steps on their site.
Make backups of your files before they are gone, what if your phone was lost or stolen or simply dropped and broke?