The screen is already telling you where the chain broke — in words
“No signal” is not a description of a broken television. It is a report about the chain in front of it, and the chain has five links: the source (cable, fibre, satellite, IPTV), the set-top box, the cable, the input the television is tuned to, and the screen. Any of the first four failing produces exactly the same sentence on the wall, which is why people jump to the last one — and the last one is the only expensive one.
Here is the fact that does all the work: the words on the screen are drawn by the television itself. That is what an on-screen display is — the set composites its own text over whatever it is receiving. So whether text appears tells you what the television believes about itself:
A box with words in it means the set is saying “I am tuned to this input and nothing is arriving on it”. Its input circuit, its decoder and its text generator are all running. It is not broken; it is reporting. The break is upstream — the box, the cable, or the wrong input. A completely black screen with no words at all means the opposite: the television believes it is receiving a signal, so it is not overlaying anything. Something is being sent to it, and the content is black — a box in standby, a box that has crashed while still driving the output, a resolution it cannot handle. A black screen and a dark power lamp is the only case where the television is the answer.
That distinction between “words” and “no words” is worth more than any amount of cable-swapping, because it splits a five-link chain into two halves before you touch anything. And it costs nothing: the television is doing the diagnosis for you every time you switch it on.
Three faults, three answers
Words, and the box has a green light is the most common fault in the whole chain, and it is free: the input is wrong. A television has two, three or four HDMI sockets plus an AV and maybe a USB. The cable is in HDMI2 and the set is sitting on HDMI1, and the set is perfectly correct to say there is no signal on HDMI1. It happens constantly because a set remembers the last input on some models and resets to the first on others, and because a remote can change it with one stray press. Press Source and step through every input, pausing three seconds on each — the handshake takes a moment, and stepping faster than that skips the one that was about to come up. If none of them work, push both ends of the cable in until they seat: HDMI has no latch, and two millimetres short looks identical.
No words, and the box shows red or nothing is the box. Red means standby, and a set-top box that loses power comes back in standby, while the button on the remote is a toggle rather than an “on” — so pressing it once may switch the thing off rather than on. No light at all means no power: the plug, the extension lead's switch, the hard switch on the back. Green but unresponsive to the remote is a crash, and the fix is to unplug it for thirty seconds, not three. Thirty seconds is how long the capacitors on the supply board take to fall below the threshold where the reset circuit will actually fire; a shorter break leaves it remembering its crashed state.
Picture, but it tears, flickers or drops out means both ends are working and the data is arriving damaged. Long or thin cables do this, especially at 4K; so do oxidised or half-seated connectors; so does a failed HDCP handshake between a source and a display. Replug both ends first, then swap the cable, then move to a different port. There is one free test that settles it: put the same cable into a laptop or a games console. If that works, the cable is fine and the box is at fault; if it does not, the cable is.