As Gromit suggest, simply carrying out a fresh scan might solve the problem.
Otherwise it's likely that your aerial can no longer provide a strong enough signal on the frequency used by the PSB1 multiplex. (TV transmissions are in 'bundles', called multiplexes, with each multiplex using a different frequency. All of the BBC channels, other than the HD ones, use the PSB1 multiplex. All aerials are better at some 'pulling in' some frequencies than others and signal strengths are affected by all sorts of things, including atmospheric pressure, precipitation, solar flares, etc).
If a fresh scan doesn't fix the problem, buy one of these
https://www.argos.co.uk/product/9383027
and one of these
https://www.screwfix.com/p/fly-lead-coaxial-plug-to-plug-1m/41720
instal them, rescan again and all should be well.