My Connect won't 'boot' and does not show up in the controller's list

My Connect was working perfectly but then dropped off the controller list of 'rooms' or 'zones'. The white light was not showing.

I have tried to reboot it by switching off for several minutes but when I switch it back on the white light comes on for approximately 4 seconds then goes off. No other colour lights show. I have tried the 'reset' (switch on whilst holding mute button) but it still shows the white light for 4 seconds then goes out.

I have tried with my other Sonos speakers all switch off, with the unit connected via ethernet and without, with a speaker connected via ethernet etc but no change. The unit still gets just as hot and the optical port has a red light inside and the ethernet ports both seem active when plugged in (green and orange lights)