A few things to check:

Will it work on another monitor/screen?

Otherwise:

Flash MB BIOS via flashback.

GPU installed in the top 16x slot?

Did you plug the HDMI cable into the GPU or the mainboard?

Does the mainboard go through the check or does it get stuck on a check LED (see manual and LEDs at the top right).

The Hdmi cable must be plugged into the graphics card, not into the Hdmi output of the mainboard. This is a common mistake. The mainboard also has graphics outputs, but these are only used if you do not have a graphics card installed in the PC. Everything should work on the 4090.