Why does my screen flicker once turns off after digitizer replacement?

I attempted to replace the screen of a IPad mini 5. After testing, I noticed the screen blinked/flickered once and then turned off. I did a force restart and tested the iPad on the old screen and the same thing continues to happen. Plus while Repair I noticed I forgot to disconnect the battery cable… Please assist. I was trying to help a friend, would hate to pay $500+ for a new iPad. Thank you.