We've always been able to get our photos when this happens. The system tags the photographer and timestamps his images, so they can usually find you, if you have someone else's photos.
It's tricky if you don't have any photos at all. Then they need info like date, time, location, and if you have the first name of the photographer, it helps immensely!
We were able to get our missing Santa photos last year from the Grand Californian, 3 days later. We always do a family photo there, as we have one of our wedding photos of us sitting in Santa's chair. For me - it was a minor tragedy when those went missing and I was going to have them back, come hell or high water! Since the PP booth had already packed up for the season, a quick trip over to the DCA counter at Postcards from CA proved highly successful!