I had made a survival world and had been playing on it for at least a week. Today i went on it and i realised i lost the progress i made and was back to the 1/2 day of playing it, my inventory was back to how it was at the beginning and i didn't have any of the structures i had built. (i had built a house, garden had pets and a mine etc...). Earlier on a similar thing happened and all the pet wolf's i had disappeared even though when i last saved they were all with me. I had always saved before exiting the game and everything else is working perfectly fine. I had full iron equipment and had the basic achievements unlocked.