What can I search to repair in my board?

I have this tablet, i used it a lot...but after one year the battery inflated, so i removed it and added some wires to put external batteries...but...one day i accidentally connected a wrong pack with 9V instead of my usual pack of 4.2V that is the voltage that it needs. I tried to find the main problem in hope to fix it without change the board because i don't have money for that. I noticed that it have some energy in every part, and when i press the power button the voltage increase and drop quickly. can someone help me?