You have nothing to worry about, for some reason this happens all the time with the itv channels, im not completely sure why, i guess itv are updating the channels.
It seems to happen every couple of months for about a week at a time, if you have a good television it will tell you there are some new services available and that you need to re-tune, otherwise just re-tune every so often till they come back.
I have had the same problem twice now.
Thank you all for your comments. I have retuned with no success and in fact on one set have lost all ITV signals completely! This has also happened to another family member living nearby.