LG phone is showing "unfortunately system UI has stopped". Background is black

[Android Central Question]

cannot switch off or reboot and no access to home screen.

 

[B. Diddy]

Welcome to Android Central! Which LG phone? If it has a removable battery, pull it for a few seconds, then reinsert it and try powering on again. If the battery is nonremovable, then press and hold Power for about 20 seconds to force a shutdown, then power back on and see if the same thing happens.