No power (no sound, no screen, LED on Nintendo board not on)
1. Verify the voltage at the Nintendo PCB battery terminals. This is nominally around 3 volts, but may be slightly more or less. If it's zero volts, check the battery, the type C board, wiring and/or soldering, or diode direction (if GBA).
2. If you have voltage going into the Nintendo board, the most likely causes of no power are the power switch or a blown fuse. These fuses are very small and can be checked with a multimeter for continuity (resistance should be zero).
Try stock charging
Verify wires are correct on type C board
Verify wires are correct on backside of connector
Try stock battery