Most people would post in Technology (or possibly in How it Works).
However the first thing to do is to check that there's not any engineering work (or known fault) with the relevant transmitter:
https://ukfree.tv/maps/live
Then look at your TV aerial to see whether it's been knocked out of position by high winds. Also check any connections in the aerial lead that you can access.
Remember that the signal strength at your aerial doesn't remain constant. It varies according to such factors as the weather, atmospheric pressure, the 11-year sunspot cycle, individual solar flares, local interference from devices in your own home (such as computers, monitors, printers, etc), similar interference from your neighbour's houses, other interference from radio transmitters used in vehicles and from 4G phone masts, etc, etc.
Further, the lengths of the elements of your aerial should precisely match the frequency of the incoming signal. However different multiplexes (= bundles of channels) use different frequencies, so any aerial will be better at receiving some multiplexes than others. (i.e. it's not unusual to have an aerial that can pick up some channel groups easily but which still struggles to receive others when reception conditions are poor). Such aerials often require the use of a signal booster to ensure reliable reception across all channels. (So you might need to consider getting one).
Also check that there's not new 4G activity in your area:
https://at800.tv/
There are a few starting points for you to consider - and that's before you've even posted your question ;-)