Tamed mule despawned

I was playing my game yesterday I had bred my horse and donkey to make a mule so I could put a chest on it and today whilst starting up my game my mule is gone! With all my stuff in its chest! I had stocked it up with food and my enchanted tools ready for going out and exploring my world. This has never happened to me before I only have 1 horse, 1 donkey and some farm animals. Never had any of my animals despair on any of my other world's until now :/. Please help!