I'm afraid that the power surge has permanently damaged some of the insides of the phone if it still doesn't work after the steps you took. I would very carefully open it up and look for blown capacitors and other areas that look burnt. If you see any failed components, replace it with the correct size and it should work. Also look for some internal battery that may still be powering the phone and remove and replace that. Hope this helps! If all fails, you may have to buy a new mainboard.