Hi
If your GoPro isn’t turning on I would try a good SDcard (that’s compatible)and if you have tried that ,open it up and have a look for anything strange in the main board
Keep a eye out for corrosion as it’s quite common on GoPros as people sometimes forget to put the waterproof housings on them
Here’s a guide on dismantling it
[guide|50697|new_window=true]
If you see any corrosion try cleaning it away with some isopropyl alcohol
If it’s all clean on the main board I would start looking at the flex cables and if they are all good try plugging it into the GoPro Quik app as it might just need a update
Hopefully this helps
Any questions please ask
Thanks:-)