You most likely require hdmi_force_hotplug=1 in your /boot/config.txt
See: Assembled my picade. screen not working. where do i change settings?
Granted this is documented riiiiiiiiiiiggghhhhht at the very bottom of our build instructions, perhaps it should be promoted to the top (thoughts, @sandyjmacdonald?) : https://learn.pimoroni.com/tutorial/picade/build
If you’re a Raspberry Pi user, we’re currently recommending RetroPie as the operating system of choice to run on your Picade.
There are excellent instructions on how to install and configure RetroPie on the RetroPie website.
After installing RetroPie on an SD card, and before booting it up, make sure you edit your /boot/config.txt and add or uncomment (remove the # before) hdmi_force_hotplug=1. Since the display is powered from the Pi it wont be properly detected at startup, so this is required to make it work.