I had the same problem a couple of month ago.
The receptor arm in the centre of my sattelite dish had been banged slightly out of position by a scaffolding tube.
I repositioned the arm.(only a slight reallignment) and got all my channels back.
You'll need someone to watch the telly while you're fiddling with the dish.
Also go into sky services and check the strength of your sattelite signal, it should be about 2/3s along the length of the bar. Hope this helps.
Billy
February 2008