Why won't the phone turn on? Only red blinking led when plugged in.

I fixed the display of my cousins Sony Xperia X. It worked great however I didn’t stick the battery and the backcover on. So when he let it fall, the battery fell out. He put it back in, but it wouldn’t boot anymore. The only sign of life is the led blinking red, when the phone is plugged in. Since I had other parts on hand, we found out that the battery was still working and the display isn’t broken either.

Does anyone have an idea where the problem could lie?