I can not tell you how to repair without starting what what I would do first.

FIRST, try the old generic reset. Remove charger, unplug big battery then press and hold the power button for 60 seconds. Release, install big battery, apply charger and test.

If this fails my bet is you need a new motherboard. Maybe a new CPU as well. Working on a hot (power applied) computer is dangerous and has resulted in a lot of dead boards.