You should first try removing the battery and plugging the camera into your computer USB and seeing if you can get a response. If you do, perform a manual update with the camera plugged into an external power source (no battery).
If that doesn't work, the next step is to contact GoPro Support
https://gopro.com/help/ContactUsIf they are unable to get you going, there is one more thing to try.
Freeze the camera (supposed to help "heal" soldering joints).
Remove the battery and SD.
Wrap in paper towels.
Put in sealed bag (zip lock)
Leave in freezer for 4+ hours
Remove, dry, and plug into computer USB
