Used to happen to me a lot, the dish has more than likely moved, the weight of the snow and a bit of wind or knocking it to get the snow off has more than likely put it out a degree or two, knock on a neighbours door and ask if their sky is working, if it is the you know the fault is with your dish/box.
Could also be the lmb has got wet as the snow has melted.
Have you tried turning it off and on again? if not just switch it off at the wall for 10 seconds then try it.