How to tell what's broken on a phone that's not booting

Today I got a battery and a screen replacement for my iPhone SE 1st gen and followed instructions to replace both.

At the end of the procedure, before pushing the display back in I booted and everything was working fine, the new battery was at 30% and the screen, camera, proximity sensor were all working.

I then shut the phone down and pushed the screen in, but it didn’t fit, it was slightly too big and one corner of the frame had a slight dent. so I used my sucker to pull the screen back out in order to fix the dent in the frame, but I pulled too hard and instead of pulling down close to the home screen I pulled at the top, this moment I think I managed to damage something, because when I fixed the dent and put the screen back in (with a lot of applied pressure, this is the other moment that I suspect I might have broken something) the phone didn’t boot anymore.

I hope I only broke the wire from the screen, but the phone gives absolutely no response when I plug it in or when I try to boot it.

Is there a sure way to tell what’s broken? if it’s the motherboard or the screen or whatever?

I still have my old cracked screen which I could try placing back but I first would like to know if it’s really the screen or maybe something else.