Replaced battery and get no response when charging or turning on.

Recently I replaced the battery on a customer’s Samsung tablet. Before the replacement they said it had been left sitting for 6 months, and when they picked it up again it was dead. They took it in to Samsung and they were told the battery needed replacing. After the replacement I still got no signs of life out of it when trying to turn it on or charge it, just a black screen. I tried opening it up again and making sure everything was connected, I held down the power button, I pushed the power button and the volume buttons at the same time, and I plugged it in to my computer (which didn’t even recognize that it was there). I have absolutely no idea what to try next. Could someone give me some advice please? Thanks!