SOLVED: HP EliteDesk 800 G5 PC that is stuck in a boot loop

[KirklandUser]

I have an HP EliteDesk 800 G5 PC that is stuck in a boot loop. The PC completes its Power-On Self-Test (POST) and gives a single beep, but the screen remains black with a steady, non-blinking dash in the top-left corner. I cannot get into the BIOS menu or the boot menu to change the boot order.
I have performed a full CMOS reset, including removing the battery and draining all residual power by holding the power button. I have also attempted a BIOS recovery using a bootable USB stick and the Windows + B key combination. However, none of these standard methods have been successful.
I believe the issue may be related to a security feature in the BIOS that has been inadvertently enabled or corrupted, possibly triggered by recent changes I made in the Secure Platform Management (SPM) settings. This is preventing the PC from booting from any drive, even a known-good, bootable USB stick. I am looking for a procedure to override this security state and restore the BIOS to a working condition. Thanks for any help.

UPDATE : Sep 22, 2025 Problem solved. What appears to have happened is that I turned off Secure Boot in the bios and doing so required that I eventually enter a 4 digit confirmation code in order to acknowledge that I really wanted to disable Secure Boot. When it came time for the BIOS to prompt me for the code, my monitor never displayed the prompt, which left the bios in a state whereby it would not return to normal operation before the code was entered. The bios not returning to normal operation meant that the bios would not respond to F10 being pressed, so that effectively locked me out of the bios. I took my PC to the second hand store where I purchased it and explained that it would no longer boot up or give me access to the BIOS. The technician took my machine away to his workshop. When he returned, he stated that the Secure Boot protection had been turned off and he was being prompted to enter a 4 digit code immediately after he powered on my machine. Once home, I asked GEMINI AI to explain why my BIOS worked in the repair shop but not at my home. GEMINI said that that my monitor worked OK while I was navigating in the bios at home, but when I exited the bios and the bios was left in the state of waiting for me to enter the 4 digit code, the handshake between the bios and my monitor failed because the bios video driver became less tolerant of my monitor (a problem due perhaps to my passive DisplayPort to HDMI cable or mismatched video resolution or frame rates). GEMINI believes that while the technician in the repair shop was able to enter the 4 digit code, thereby completing the deactivation of Secure Boot, the bios immediately re-enabled Secure Boot. I asked Gemini why the bios would re-enable Secure Boot after I had ordered it off. Gemini said that all my attempts to unlock the bios had made the bios suspicious of the user and the bios decided to revert back to it most secure mode, which meant turning the Secure Boot back on. So I now need to discover what was different about the monitor and video cable in the repair shop compared to my own monitor and video cable at home.

[KirklandUser]

Question: has anyone had problems displaying the BIOS's prompt for the 4 digit confirmation code after connecting the DisplayPort of their machine to the HDMI port of their FULL HD monitor while using a passive video cable with a DP connector on one end and an HDMI connector on the other end?

UPDATE: Sep 23, 2025 Today I found that if I used the same passive DP to HDMI video cable with my much older monitor, then my BIOS had no difficulty displaying on the old monitor during the most sensitive operations that the BIOS was running. My much older monitor had a DVI input, and fortunately I had an HDMI to DVI adaptor. It would have been nice if my brand new FULL HD monitor was not failing at this task, and someday I might buy an active DB to HDMI video cable just to see if that enables my FULL HD monitor to work as well as my old non-HD monitor.