If it has a hidden or obvious Bios backup battery/coin cell, keep reading...
Put all the components back where they were,
check the DVD drive is empty,
plug in a spinning hard drive not SSD,
attach a USB keyboard and mouse in the USB 2 slots only, don't use the blue ones
pull out the Bios backup battery/coin cell
wait a few minutes or longer
Start up the HP without the battery and see what happens
If nothing happens, pull out both ram sticks and alternate one at a time in the ram slots and cold boot PC every time
Keep the battery out during this possible futile time waste and check it for 3 volts using Low-Z on the meter
You may get lucky as I have a few times with zapped AIO PCs