Sounds like your fish went on a permanent vacation and his tank mates helped him!
Were all the fish about the same size or were there some much bigger than others?
In fish world, I'm sorry to say but the bigger fish do eat the smaller ones sometimes.
If you read the ingredients on the food you feed your fish...well goldfish are not particular of what kind or brand of fish they eat and probably the fresher the better.
Sorry for your loss.