The Ryde is plugged into the display and works fine.
The display device is switched off but the Ryde is left on.
The device is switched on and there is no output from the Ryde.
Switching the Ryde off/on should not be done by power off/on as this may upset the firmware (like most computers).
Log into the Pi using ssh and reboot or power off then off/on the power or power off using gpio and power off/on. This seems to restore the output. Reception of a signal may also start the output working.
The Ryde is working all the time, it is just the output that is confused. Not sure if it is the Ryde or display device but setting the display device to full HD cured it. The display device could be switched off and on and the Ryde always had an output.