I went into the System UEFI setup and reset to defaults. Upon restart the machine began an endless cycle of powering on and off. The HP splash screen appears only for an instant before it shuts off and turns on again. I can't get into the BIOS again.
Clearing the UEFI by motherboard reset button slightly changes behavior by causing an initial longer startup, but then it resumes the power on/off looping.
Password jumper was missing. Replaced it but no effect.
I noticed before resetting to defaults that the system UEFI is a 2016 version so fairly old.
Any ideas? Is it history?