Electro Magnetic Pulse
Can be generated by nuclear explosions, either from the Sun or Man
Defence Industries have spent billions since WW2 to alleviate the effects of EMP, mainly because they had an impact on the employment of nuclear weapons:
It affects the whole frequency spectrum, but the longer wavelengths, VLF, LF and HF suffer most, usually days and weeks.
With Higher Freqs and shorter wavelengths the effects are are considered temporary and last for hours or days.
By adjusting the MUF, MUA and power output based on the height of the Ionisphere, most HF/DF comms can be re-established fairly quickly these days.
With SatComm it's all changed - however, if you were at ground zero during a 10 Megaton airburst, you might have problems sending a text on your mobile