The dreaded "support.apple.com/mac/restore"

I have a 2021 24 inch iMac with the M1 chip. Two days ago when I tried to switch it on I got the screen with an exclamation mark and the message "support.apple.com/restore".

I have read the advice online, which involves using second Mac (I have a MacBook Air 2019 running Sonoma), but when I try to connect the two, nothing happens. I have tried all the ports of my iMac (the advice seems to be to use the one on the right as you look at the back of the computer) and am using a USB C cable capable of data transfer (it came with my iPad Air). The instructions say to hold the power button for 3 seconds when switching the defective Mac back on, but this is not working. Nothing happens - the defective Mac does not appear in finder (or Apple Configurator) of my MacBook Air.

I cannot start the iMac into Recovery mode either, so any advice would be gratefully received.