If there's anyone with a T3620 with bios revision 2.16.2 (or later) and could record something like this:

https://www.youtube.com/watch?v=YcxxMwyw_T4

but from pressing the power button.That would be VERY appreciated

Have you tried to contact ProSupport ?

If you just purchased it, you would be entitled to their help

Secure boot doesn't post for non dell video cards. This is true for ATI as well as NVIDIA and other vendors.

https://nvidia.custhelp.com/app/answers/detail/a_id/3156/

When an after-market graphics card is installed into a motherboard with UEFI enabled in the system BIOS, or if the system is a certified Windows PC with Secure Boot enabled, the system may not boot.

1. Secure boot is designed for not-booting when the system was tampered. And that is exactly what you did.

2. OS recovery option will boot to bios after N-failures, so it is not necessary battery-resetting BIOS if you have enabled that option.

3. After battery, or OS-recovery mode, the non-DELL GPU will be recognized by board, the display will light up... It may. I had success with Gigabyte GTX 780ti updated with its UEFI bios. I had no success with nVIDIA Quadro FX 800. It has no UEFI code in its bios. But I may have been not patent enough to wait for the "recovery" boot...

4 Quadro RTX 4000 supplied with Dell WSs seems to be unrelated to Dell. These are just nVidia boards. And those boards boot well.

You should contact Dell support as suggested by another member on here.  Practically most problems can be resolved if you know the cause.  I have no information that could help with your issue but for the sake of learning, I just ran some test to verify the information given by others on here.  I pulled a HP stock graphics card GTX 1070 from a HP Omen desktop and I installed it in a Dell Precision 5810 tower.  With secure boot enable, the system posted just fine.  Shut it down and put back the Quadro M4000, It is running without any problem.

With the result, less likely that your graphics card was the cause.  While waiting for Dell support, have you try booting without your SSD?  If not, install your graphics card back on and removing all storage cables and SSD.  Hopefully you can get your display back and restore your BIOS settings.

I have the same issue and checked all tutorials from DELL to reset BIOS to default 

https://www.dell.com/community/Precision-Fixed-Workstations/Where-is-Dell-Precision-Tower-3620-recovery-BIOS-stored/m-p/6147810#M806

https://www.dell.com/community/Desktops-General-Read-Only/Clearing-the-CMOS-NVRAM-on-Precision-Workstation-Systems/td-p/2575613

but still, no DISPLAY while keyboard lights are ON.

I am using zeon processor and 3rd party GPU.

I have changed the processor with intelligence and access to display successfuly and in bios settings DISABLE secure boot options and then switched back to zeon processor and everything is workable.

Are you saying that you removed and re-installed the processor and you were able to get to the UEFI screen?

Yes but bot the same processor.

1-Removed zeon processor 

2-Plaved Intel processor 

3-Access the BIOS display and settings then "disable secure boot options "

4-Revert back to zeon processor 

That's fine

Shouldn't there be a way to do this without using a different cpu ?

Yes there is if you use dell bios supported GPU card since zeon processor has no built-in VGA display functionality. 

Ok,

but once I buy a computer, is mine, and is my own business what I put inside. If I'm unable to reverse something just because they didn't plan enough well for this situation, the thing falls on the design , not on the client.

I can accept that it would not work with "secure boot" , if the part is not signed. I cannot accept that the only workaround is (clearly by the thread) to either use a different cpu or to buy a signed gpu.

If the client has to bear extra costs, those should and must be totally on the maker. Be it a new unit, or an used one.. since this clearly is showing a design flaw preventing a reset.

Personal opinion, anyway

i know this is old, but just in case...   I made the same mistake. It took me hours to figure it out.  I removed the battery, removed the graphics card. removed power for 5 min. Nothing worked.  Then I remembered the t3620 with xeon has no internal graphics card, so duh, using hdmi port is useless.  I was using the display port on the nvidia graphics card. I unplugged power, removed battery,  pressed power to ensure no reisdual power, then  I put back graphics  card and  switched to dvi port.   Del splash screen, system gave boot options and I was able to switch back to no secure boot and legacy boot.    What a waste of time because intel and microsoft want to push latest h/w and s/w

Same problem: T5810 with nVidia card, no on-board graphics; no video after enabling Secure Boot.

"jefinsd" led me down the right path: clear the BIOS with the graphics card out, reinstall the nVidia card and use the DVI with a VGA cable and I got into the BIOS.  Thank you, jefinsd!