Uly, as a puppy, honestly had the waggiest tail known to dog or man. So it was a shock when one day it went limp and just wouldn't do more than a little shake from side to side. It was obviously causing him some distress too.
I was really worried that I my lovely dog was going to be left without a wag! Fortunately it wasn't a permanent state of affairs and within a few days his tail was back to its old self.
He'd suffered an affliction know as frozen tail (also limber tail, cold water tail and cold tail syndrome amongst other names), and it had come on after a particularly cold evening sitting out in the field. He seems to be susceptible to this and has been similarly trouble after swimming in cold water and after being out in the snow.
I do feel sorry for him when he gets it, as he's clearly troubled by it. Vets can give an injection to ease the discomfort, but this could get expensive if it's a regular occurrence. Best thing it seems to me is take it easy on the kinds of activities that cause the problem.
UPDATE: Poor old boy was out in the snow and cold yesterday and came back with a cold tail and and a very down-in-the mouth look. At first he kept nibbling away at the fur on his tail, as if he had a flea – I think it probably feels like chilblains and gets very itchy. This morning we're going to take him for just a short walk (so he doesn't get too cold) with his mum – the activity will set him straight for a while and then he can chill-out for the rest of the day in his bed. Hopefully he'll be back to normal in a day or two.
LATEST: Took Uly up the hill today (snow on the North Downs, lovely blue sky but very cold). His tail, while not entirely back to normal wagginess, is considerably better – well on the mend, it seems.