Phone is unresponsive unless plugged in after battery replacement.

I replaced the battery on my iPhone 6. The phone was working great, but a day after the replacement, it stopped working altogether. Nothing will work. I opened the phone back up and checked the connections with the battery and screen connectors, and they are all secure. I tried plugging the phone in, and after a few minutes, it powered on like normal and gave the usual two vibrations to show that it was charging. However, after that, the screen went black, and it will not turn back on again.