*Edit*
Its a player issue. you probably have all set down to the lowest. its ok to keep it that way but you need to change the graphic execution mode to compatibility (you should have it on directx, that is the problem)
happened to me too , i'm using bluestacks , first time it happened i just tuned the graphics and they all "reapeared" , oh and if you examine carefully the mobs/characters/npc's didn't really disappear , they're just either floating high up or sank underground that's why you can't see them