UKSomething has stopped working
Start with what changed. Which channels went, and whether they went all at once, is usually enough to tell a fault at the transmitter from a fault at your end.
Some channels have gone, the rest are fine
Then a multiplex has gone, not a channel. Freeview's channels travel in six bundles, and everything in a bundle arrives or fails together — so the fastest way to confirm it is to look up one of the missing channels and see whether its bundle-mates went with it. Every channel, and the multiplex it is on.
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The six are PSB1, PSB2 and PSB3 — the public-service bundles, which reach almost every transmitter in the country — and COM4, COM5 and COM6, the commercial ones, which are only carried by the ninety-odd main stations. If the missing bundle is a COM and your transmitter is a relay, they were never there to lose: that is not a fault and no aerial will fix it. PSB3 is the HD bundle and the only one using DVB-T2; a set-top box old enough to predate it loses that one and keeps the rest.
Everything has gone at once
Everything failing together points at the path rather than at the broadcast — the aerial, its cable, a connector, an amplifier that has died, or a transmitter that is off. Retune before anything else, because a set that has stored an empty list will keep showing an empty list after the signal comes back.
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Amplifiers and their power supplies fail more often than aerials do, and they fail silently. A distribution amplifier feeding several rooms takes every room out at once, which looks exactly like a transmitter fault from inside the house. If one set works and the others do not, the amplifier or the splitter is where to look — the aerial is proven by the set that still works.
Transmitters do go off, usually for planned engineering, and usually at low power for a few days rather than off outright. This site does not carry engineering announcements — it is built from Ofcom's published parameters, which describe what a transmitter is licensed to radiate rather than what it is radiating this afternoon.
It comes and goes, and weather seems to matter
That is a marginal signal, and it is the one symptom this site can actually speak to: look your postcode up and see how much margin the model gives you. A prediction close to the threshold is a picture that high pressure will take away. Find my signal.
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Settled high pressure bends signals further than usual — a lift — which sounds like it should help and often does the opposite: distant transmitters on the same channel arrive at the same time as yours and interfere. Rain does the reverse at UHF and matters much less than people expect. Either way the fix is margin: a better-sited aerial, or a transmitter with more of it. The lookup ranks every transmitter that reaches you, not just the nearest one.
It started on a particular date
Retune first — channel allocations do change, and a set only notices when told to. If your equipment is older than about 2013, the 700 MHz clearance is worth reading: television left the top of the band in 2020, and filters and aerials sold before it are filtering the wrong frequencies.
The picture breaks up into blocks
Blocking is a digital signal arriving with too many errors to correct, which is the last thing that happens before it disappears entirely. It means marginal, not broken — the same advice as the weather section above.
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It can also mean too much signal. An amplifier close to a main transmitter can overload the tuner, and the symptom is identical to too little. If you are within a few kilometres of a high-power site and have an amplifier, try the set without it.
Radio
FM degrades gently — hiss first, then stereo dropping to mono, which is a set doing the right thing rather than a fault. DAB does not: it is clean until it is not, and then it burbles or goes silent. Both are worth looking up, because the transmitter carrying a station is often not the one carrying your television.
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A DAB station is one service inside an ensemble, and the whole ensemble arrives or fails together — the same structure as a television multiplex. If several stations went at once, look for what they have in common: every ensemble and its line-up. FM and DAB coverage from one mast can also differ sharply from its television coverage, because the two are radiated at different powers and often in different directions.
What this site can tell you
The lookup names every transmitter predicted to reach your postcode, strongest first, with which way to point and what the terrain in between looks like. The register has a page per mast: what it radiates, at what power, and in which polarisation.