That was weird: most of my home wi-fi went down: it was fed by a recalcitrant FTTN broadband network over Ethernet. The 2.4 GHz & 5 GHz networks generated by the modem/router were not working at all, yet the connected VOiP phones were OK as was the other wifi network provided by my AirPort Express that connects to the router via Ethernet.
A manual restart of the router restored full service.