[TIGHazard]

Depends on the definition of dead air and the era.

If its dead air as in transmitting something that just happens to be a black screen for whatever reason, that may require somebody to intervene to get something else on air.  Usually an OB has packed up or something or a source has suddenly become unavailable but the channel itself is still "on air". Case in point:  The One Show once had a fire alarm, they showed a VT, evacuated, the VT ran out and there was a black screen for a while until presentation took over.

Prior to I believe 1991 the IBA transmitters if they lost their source would show you a blue screen generated by the transmitter, though sometimes the sound would come through but not the picture for some reason.  Some examples on YouTube.

In the digital era not sure what happens if the Freeview transmitters lose their source, but I presume its just a frozen picture.

Radio has the backup tape (which can on occasion reveal its age and the last time it was updated) due to the medium, as its usually the station itself is still "on air", just transmitting the audio equivalent of nothing.  I believe as as its an audio medium it has no other output.  Few examples where the fire alarm has gone off and cut the studio output to prevent that from going out on air, but equally some examples of where it has gone out on air and they've managed to segway gracefully into the backup material.  Example:  Radio 5.

I hate to bring it up, but I think 9/11 revealed that answer to be true. There's the very famous screengrab from WPIX New York, where satellite & cable rebroadcast feeds froze on the last image received from the local transmitter - the start of the towers falling, the transmitter of course being on the roof of the WTC. Apparently that image was stuck on air for the rest of the day until they could switch to the empire state building.