iPhone 6 doesn't boot after replacing the battery

Hey all. I replaced the failing battery on my iPhone 6. It went pretty well, but when I tried to turn it on, it displayed the Apple logo for around a second then went black. The backlight was still on though, but after maybe 20 seconds it turns off. Plugging it into a charger has the same behavior.

I tried connecting my phone to my Mac and it recognizes it, although it does say that I either need to update or restore. I am downloading the update as I type this. I am hoping the problem isn’t too bad and the update fixes it.

And if it helps, I did this replacement after waiting about two years after my battery gave up. Within those two years, my phone has been hiding in my closet.

Any help is appreciated. Thanks!

EDIT: I noticed something else. While downloading the update for my phone, after some time, the Apple logo shows up again and does the same thing as described above. I then have to disconnect and reconnect and restart the download process, only for it to get interrupted again.