No offense to shallowreef, but I would not suggest the scissors. The potential damage to the worm if not careful could be it's death. The loss of the crown is not all that uncommon and typically a sign of something amiss with the worms surroundings. Usually an indication something is picking at it, water conditions are not optimal or water flow too laminar. They do very poorly in nitrogen rich water.
If the crown has been "tossed", it can take a week or more before the worm will venture from it's tube, especially during daylight hours. Even after this time frame the crown will have only grown back to a small degree of it's original size and may not be that noticable. Check well after lights out with a dim or shaded flashlight to see if it is active at all. I would suggest not doing anything to directly disturb the worm as it may just make matter worse.
While your poking around at night, check for unwary predators that may be pestering the worm, especially if peppermint shrimp or the like have been added.
Cheers
Steve