Some mobs turn invisible, such as villagers or endermen. The invisible hostile mobs can interact with you and hurt you, but you can't interact with them. You can still see the effects of the mobs, for example the villager's green sparkle or the fire charge pattern effect that they sometimes have, but you can't see them or interact with them. To fix the invisibility you have to log out and log back in, but it doesn't fix all of the invisible mobs. If you get attacked by a hostile invisible mob, then you have to either run or leave the game and come back.