Nico Brailovsky's thought repository

If a Raspberry Pi boots but has no HDMI signal:

• Add hdmi_force_hotplug=1 to /boot/firmware/config.txt - this forces the Pi to send HDMI video even if it thinks there's no monitor connected.
• Add config_hdmi_boost=4 to /boot/firmware/config.txt - this tweaks the HDMI signal strength.

Also, remove all possible adapters (each will add a bit of noise and attenuation) and get a cable with good shielding.

Source https://elinux.org/R-Pi_Troubleshooting#No_HDMI_output_at_all

Extra tip: Unlike their bigger brothers, Raspberry Pi Zeros don't seem to want to boot up with no SD card in, not even to show a bootloader error.