Re: I currently have a monitor plugged into the DisplayPort that is most likely reason you have black screen during POST.   only the most basic VGA is giving video at the initial stage of POST.  it requires least amount of firmware.  analog only.  no digital video.  for diagnosing POST issue always use VGA when possible. Re:  I currently don't have a VGA monitor though old fashioned they are handy when you need one.  keep one around if you can.  you can often find a free 17" one locally that owner does not want any more.
After switching to secure boot with my OptiPlex 9010 (because I wanted to try it out with Linux), I have been unable boot or even see the splash screen.   This appears similar to this issue (dell optiplex 7010 bios screen will not show - Dell Community) and I left a reply there also. I followed the advice of several threads of these forums to try to reset the RTC battery, including using the reset jumper pin, removing the battery for 30 seconds while the computer was unplugged and pressing the power button, and leaving the battery out and computer unplugged overnight.  I also bought a new battery to replace the old one.  I have unplugged all hard disk SATA cables. When I turn the power on, it seems to do something.  The Num/Screen/Caps Lock buttons work and Ctl-Alt-Del reboots the machine.  I don't have a GPU card.  I currently have a monitor plugged into the DisplayPort, as there are two of these, no HMDI, and one VGA connector.  I currently don't have a VGA monitor.  There is no internal speaker plugged in, so I don't know whether there are any system error beeps happening.  I don't see any amber flashing patterns. Is it possible that only the VGA is active during this moment?  Is there any sequence that can enable some other diagnostic?  Or is the machine just bricked? Thanks in advance!
Black screen with no Dell Logo after switch from Legacy Boot