Start by checking here to see if there are problems with your local transmitter:
https://ukfree.tv/maps/live
(Zoom into the relevant area and then hover your mouse cursor over any relevant 'Men at work' icons).
If you don't see any transmitter problems, check your reception on these channels:
3 = ITV
5 = Channel 5
6 = ITV2
10 = ITV3
15 = Channel 4+1
18 = More 4
25 = ITV 4
27 = ITV Be
29 = E4+1
All of those channels share the same frequency as Film 4. If several of them are missing, it suggests that your aerial doesn't resonate well with that particular frequency, causing you to lose channels when reception conditions aren't at their best. If so, you need one of these
https://www.argos.co.uk/product/9383027
and one of these
https://www.screwfix.com/p/philex-coaxial-cable-1m/41720
If neither of the above suggestions seem to be relevant, the problem might be related to the fire at Red Bee Media
might be relevant but, given that their London distribution centre feeds transmissions to all transmitters (with other here reporting no problems), t seems somewhat unlikely to me.