Why is my logic board overheating after screen replacement?

I own and iPhone 7 plus and I had in mind to mod the back logo with an led logo and change the battery with a new screen, after it got completely shattered, but when I got the new screen I tried connecting everything in to its place but accidentally broke the home button connector and so I bought a new one and placed it.

When I started the phone all I got with the new battery is that the apple logo would light up for a good 5 sec and then shut back down. And when I tired using the old battery the logic board got insanely hot that I couldn't even touch it because it would literally burn my finger.

I've been doing some research and some suggest that I didn't power device completely down before even starting to change screen, to be honest with you: I don't even know what I did at all, Now, it wont even go to the home screen because its stuck on the apple logo.

Please help asap! otherwise ill just buy a new one