One moment you are using the phone; the next, you see a black screen with green, yellow, or white text listing options like "Reboot," "Bootloader," "Recovery Mode," and "Power Down." Your touchscreen doesn't work, and pressing the power button does nothing except cycle the screen on and off.
The KeyOne has a non-removable battery, so you cannot simply pull the plug. However, you can force a hard reset via button combination. blackberry keyone stuck in bootloader menu