Well, I found an old vga monitor, reset CMOS again, and still black screen. So I swapped back in my old MB. It worked. Maybe I bricked my “new” MB, maybe It was already bad. Hard to know. Anyway the old MB is now working without issue. I did notice a slightly bent pin in the CPU housing and bent it back into place. Maybe that or reseating all the MB connections fixed it. Hopefully so. Thanks [NAME] [NAME] the troubleshooting advice and thanks [NAME] [NAME] the BIOS boot settings, which are what I used for this working boot.
I have an Optiplex 9020. It was giving me bad motherboard diagnostic codes, so I replaced the motherboard (with the same model as was already in there). On first new start it gave me a boot error, which happens on my PC if the BIOS gets cleared. I went into the BIOS, changed the boot from legacy to UEFI, and it successfully loaded windows. Then I restarted and entered the BIOS again to see if anything else needed changing. I don’t recall what I might have changed at this point (maybe to/from raid?) but now I have no video. PC starts with no beeps or diagnostic lights, but no video at all. Not even the dell circle at startup. I’ve tried resetting the bios , unplugging my gpu, pulling RAM, nothing works. Very rarely I can get to the “set your clock” screen in the BIOS after clearing CMOS, but then the screen goes black again as soon as I try to exit the BIOS after setting the time. my setup is to connect hdmi to an asus gpu and to connect keyboard and mouse using a wireless dongle. I don’t have easy access to anything else to connect. Any thoughts? Have I tripped some Dell booby trap? 
Optiplex 9020 no video after modifying BIOS settings