So my friend and I have a Minecraft world, but one time when my friend rejoined the world, he was at spawn with nothing but a map, as if he never joined in the first place. Even when he got home it wouldn't let him interact with his cats and he lost everything. Fortunately, when he left and rejoined he got all his stuff back and his cats liked him again, but that was still terrifying and we have no idea what caused it.