you've not got a strong enough signal from your aerial. if it's been fine up to now then the recent bad weather may have effected the aerial in some way (high winds blowing out of alignment, water in the connections on it or if you've still got a lot snow it may just be that the aerial is covered in snow and ice)
I assume you checked all cables are plugged in OK (disconnect them and reconnect to make doubly sure they are making good contact)
If not it may be time for a new box (especially if it was a cheap one) I've had 3 in the last few years, the cheap ones don't last long at all (then again at £15 a time if it lasts a year I don't feel hard done by)