It is always the "no me" ghost!! My 3 granddaughters lived with me and when I would loose a piece of fabric or block I would ask them if they had seen it and the reply was "not me". Yes, I have lost a few blocks in my time of quiltiing and possibly your block has stuck to some other fabric or it has fallen down behind your machine or even your design wall. Good luck on your search and you know you will find it when you have made a new one.