IT is said that if you leave the head in the tank, it COULD grow a new worm.. I have never seen it happen..
Ya they loose their head for a few different reasons, water quality, physical stress, or sometimes the "feathers" get worn and they need to make a new one. It takes a few weeks for it to grow back but I have lost a head on my coco and on my HI featherduster and both have grown back.
If the worm ejects from its tube you got major issues. If that happends, it is said that you should bury it in the sand.