Wow, that sucks. Can you take a picture of the coral and its surroundings? I've had a few heads on my frogspawn die overnight but that was caused by it falling over onto another coral.
I can only assume that a colony as long lived as the one you have will have needed a pretty strong force to cause that much harm overnight. Any additions or changes to your tank within the past two weeks? New lights, more live rock, new corals, moving corals around, change in supplements, change in salt, temp fluctuations?