Screen only shows low battery after repair

I did a repair for a friend a while ago but it never fully completed because the screen wouldn't come on. The phone vibrates when it's turned on and touch ID works, it's just that the screen never shows anything else but the low battery symbol and that you should plug in the charger. Even after letting it charge for a while the screen still won't come on. I'm wondering if the screen is faulty or if there was a mistake I made while installing it. I have taken the connectors for the screen out and put them back in, the same thing with the battery to no success. Any advice would greatly be appreciated, thank you.