Since Windows 11 failed to install the newest security update, I googled the error message and found a suggestion to enable Secure Boot in BIOS.

However, after I enabled Secure Boot and restarted the computer, I had no POST. Only the black screen.

I tried holding F2 and tapping F12, but no Dell logo is showing up. Actually, the monitor is not getting any signal.

I tried removing the battery (even overnight), to reset CMOS, but with no success. 

I also tried to hold the Power button for 30 sec, but nothing was happening after...

When I tried holding Ctrl+Esc, I got a light-indicator error "BIOS recovery image not found" - which is expected, since I replaced the original SSD recently and I don't know where it is.

When I tried to put SSD from another (newer) Dell computer, got a different error "Recovery image found but invalid" -  which is also expected.

I tried USB BIOS Recovery - download .rcv from the Dell website by using my Service tag, then used FAT32 formatted USB. That time there was no error at all, but still no POST.

Any thoughts anyone? Thank you!!