No Logo on Boot. System Hangs, HDMI not recognised

At 0900 was running fine with previously installed version of Win 10 2004 (first iteration) but I wanted to do a clean install of Win 10 2004 with all latest fixes.

Tried but could not get F2 (or F12) to work to allow me to change UEFI to boot from CD. Called Microsoft (wrong move!) and system is now inoperable as begins boot but then nothing shows - no Dell logo, no errors etc on screen and system hangs.

Checked HDMI cables and screen, both working fine on my laptop. Spoke to three different MS employees but no joy so far - anyone any ideas? Would happily reset all and sundry as have backups but how to get past initial hang to allow update ?