Hi Mike, Putting my angling hat on, heres a serious answer for you !! I would imagine if, as has been said already, you havent seen floating dead fish, that they are fine and as soon as the temperature rises and the water clarity improves, i'm sure they will 'reappear'. Goldfish (Carassius auratus) are indeed a member of the carp family (cyprindae) and while they dont hibernate in the true sense of the word, cold weather will send them deep, where they will remain fairly inactive until the water warms again. Of course, it is possible that they have been predated by a heron but my gut feeling is that is not what has happened here. Fish actually move fairly well in winter and just for future reference, the worst time to move fish is during red hot weather (when oxygen levels are lowest in water) and never when its really thundery.
best wishes Lannerman.