To shut-down the device, a long press of power button is necessary : blue light. A short press : standby mode. An intermediate action : power on.
For me, it's the clock screen that block the device...
Retry the long press to obtain the blue light. Wait ... wait .... Press the power button.
If all this don't succeed, shut-down the device and disconnect/reconnect the battery :
Disassembling of Onyx M92