Loss of certain channels (particularly the ITV ones) is a classic symptom of an inadequate aerial system.
In an ideal world, you'd have an aerial where the elements were exactly the right length to 'resonate' with the incoming frequency for each channel. The only problem is that you'd need a separate aerial for every channel you watch! All aerials only offer a 'compromise solution', which means that they're better at receiving some signals than others. Those signals broadcast by the ITV companies seem to be the hardest ones to get a good signal from. (Another AB contributor has stated that they use a different type of encryption, which requires a slightly stronger signal).
If you used to get a good enough signal, but now don't, the cause
might be entirely external. For example, radio signals on earth are affected by the the 11-year cycle of activity on the Sun's surface. However, it's far more likely that your aerial system was originally of 'borderline' quality but has now got slightly worse.
The first thing to do is to step outside and take a look at your aerial. Check that it's not been blown out of position by high winds. (If so, you need either a ladder or an aerial fitter).
Otherwise, call in an aerial fitter to assess your system. To get a really consistent digital signal, many people find that they need a new aerial and (equally important) new cabling. If you need a new aerial, this one has been widely praised, both here on AB and on other forums:
http://www.maplin.co.uk/Search.aspx?criteria=P T18U&DOY=29m2
Chris