Why is my Go Pro Hero 12 not switching on . Battery fully charged

@viviennemetz besides telling you its fully charged, have you checked the battery with a meter just to make sure it is. Apply a load to the battery and see if it stays functioning.

Follow the instructions from GoPro "Disconnect the camera from any external power source and remove the SD card. Remove the battery, wait five seconds, and reinsert the battery. Press the Mode Button to see if the camera responds (look for LED illumination, beeps, or a response from the camera's screens)" More about that on here

If none of that works, you know it is on the inside of your GoPro. Could be a bad ribbon cable, corrosion or many other issues. You may have to disassemble it and take a look at anything odd.