You will be aware of thunderstorms in parts of the country. Most relays do not have generators therefore if the power goes off, so does the transmitter. Alternatively, and possibly more seriously, the transmitter itself could have been struck by lightning and damaged.
The Cop Hill relay of Emley Moor here in Huddersfield went off during the thunderstorm at about 4.30, phoned BBC Information to let them know, still off at about 7 when I checked.
I was watching at a friend's house in Alnmouth (Northumberland). Channel Five went off, so did the 'spare' channels he had tuned in to a more distant transmitter. I think was Bilsdale or a dependent because he mentioned they used to get Tyne Tees South news on the spare 'ITV' channel.
Slightly OT: But can the bad weather affect Digital Cable? We've had no NTL at all since 4.00pm yesterday, you have to like them, we tried ringing them and all of their phone lines were shut!