HP Z440 no display output after changing to secure boot

[HPLapUserDV7]

I can not get into bios. There is no display output.

I bought a second hand HP Z440. It has a SSD with Windows and I wanted to write over it with a fresh install. It was in legacy boot and I wanted to access my USB key with Windows, so I changed to secure boot and set it so that USB boots first by order of priority. Then I rebooted

From that point I got no display. I tried unplugging power, holding down the power button for 30 seconds to discharge the capacitors, and then pressing the yellow CMOS button for 10 or 20 seconds. 

Now the system goes through a cycle of turning on and then off again with a static kind of house. I have tried removing everything, including the RAM, cards, GPU. Same results. I tried putting them back. Same results.

I am typing this on the same monitor I used with the HP Z440 but with my other machine.

EDIT: The HP Z440 has a K2000 card which I think is not UEFI compliant. Is that the reason it does not display when I am in secure mode, so it is stuck?

[HPLapUserDV7 · Accepted solution]

You need a UEFI compatible card. My K2000 card was not UEFI compatible so I got a blank screen when I started the machine.

I swapped a P4000 into the machine and I got the screen asking me if I really wanted to change secure boot. Got past that and it told me that the system had been factory reset, from my holding down the CMOS reset button. Secure boot was turned off. I was able to swap back the K2000 card and boot as normal.