Chances are it is just stressed .. They will "molt" thier feather crown when stressed ... It will grow back . keep water conditions pristine and keep anything from picking on it . Gently press the tube , if you feel him in there place it in the tank tube and all (less the crown) the new one will grow back with in a month or so and will likely be smaller than the origional one ..
They will not come out of the tube generally when they have no crown so you may not see it for a while ... Just keep testing your water for the preameters .... The other thing is that if stressed enough they will leave thier tubes this is more critical and they are likely to not survive if this happens but they can survive this ... They are really weird looking and look like a giant centipied in your tank ... HTH