One of the major causes of issues with the Zega Mame Boy Advance SP kit is improper soldering. Consult the GPIO pinout and try reflowing all pins associated with your particular issue including:
As stupid as this sounds, a number of customers have been using a Pi Zero, mistakenly thinking it to be a Zero 2. This is actually not that stupid. If you are not familiar with the Raspberry Pi Zero it can be easy to mistake one for the other. If in doubt, ask me first and I can confirm which model you have.
Check the ribbon cable is orientated correctly and try reseating. If in doubt please email me.
This means that while the power supply to the LCD is good, the video signals are not. Check all video signal solder points according the the GPIO pinout