Try a long-press on the rear power button 20s or so, until you see the status LED light up… if no joy then place on charger for approx. 30 mins and try the long-press option again.
@Ken_Griffiths thanks for the suggestion, however tried both and still no joy. Nothing happens when I press the rear power button that long.
This is more for anyone else that has the same problem:
I contacted Sonos support and they advised to factory reset the unit. This I was told is done by:
Holding down the Play/Pause button
Then pressing and quickly releasing the power button
Then continue to hold the Play/Pause button ~20 seconds.
However, for me this didn’t fix my problem so I think I have a fault.
This is more for anyone else that has the same problem:
I contacted Sonos support and they advised to factory reset the unit. This I was told is done by:
Holding down the Play/Pause button
Then pressing and quickly releasing the power button
Then continue to hold the Play/Pause button ~20 seconds.
However, for me this didn’t fix my problem so I think I have a fault.
The factory reset options are listed here:
https://support.sonos.com/s/article/1096
However in your particular case I would ignore the FR process and try to reset the PID by long presses (15-20s+) on the Riams rear power button intermingled with placing it on its charger for an hour and try rinse/repeat, until it hopefully springs back into life. If you have a 5v/3A USB charger give that a go too.