I checked when you posted last night and I've checked again now: There are no reported faults (or engineering work) for the Emley Moor transmitter:
http://www.ukfree.tv/maps/live
So your problem is most likely to be related to your aerial. e.g. it could have been knocked out of alignment by the wind or there could be a loose connection somewhere. (HD requires a greater signal strength than SD so, if the problem's not at the transmitter, it must be with the way you receive its signal).