homepods suddenly stopped working with Homekit

For some reason, this morning all three of my HomePods have stopped being able to control my devices in HomeKit. Everything was fine yesterday, but today if I ask any of them to, for example, switch on or off a light, it says “it looks like you don’t have any devices set up”. Bizarrely, none of them will set any timers either (and even more bizarrely, my Watch has stopped wanting to set timers this morning too - it prompts me to download a timer app instead??)

Any idea what’s going on?