Hello @GrayIT, thank you for your post which has received very helpful feedback from the other IT Pros.  See below the input from the consultant.  Since this is your first post, a warm welcome to Spiceworks.

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HP EliteDesk 800 G1 SFF Business PCs Opens a new window Opens a new window

Is the system clean and all fans are spinning?

Are you getting any beeps and LED codes? HP Desktop PCs - Computer Does Not Start and Emits an LED or Beep Code Opens a new window

And also page 237, POST numeric codes and text messages, see below.

I would first clear and reset the CMOS, page 251: Maintenance and Service Guide Opens a new window HP EliteDesk 800 G1 Tower HP EliteDesk 800 G1 Small Form Factor HP EliteDesk 800 G1 Ultra-Slim Desktop

Then plug in only keyboard and monitor and try to enter BIOS setup via F10 key, page 180 above. If you can't see normal BIOS screen then you have some hardware problem or your BIOS is corrupted. Try to reseat all plugs and connectors, memory etc. and try to update BIOS from USB disk. #####