iPad shows no battery but still works?

Hello, a few weeks ago I bought a cheap iPad Air 3rd generation from a friend. Since it had a very weak battery, I had it replaced at a cell phone store. The guy there was really unmotivated to do it, which was very strange. I got it back after a week and then it crashed 1-2 times. The battery didn't get any better. That wasn't really a problem though because it wasn't crashing as often. But since yesterday it hasn't shown the battery level. I charge it and waited overnight but nothing happens. When it's charging, the start screen says it has 0% or it's not charging at all. It does have power though because it works and I can use apps, but only for a few minutes because it crashes. It's happening a lot at the moment and I can't use it anymore. When it crashes, the screen freezes for a moment and then briefly lights up with either a pink or green light.

Please help, I need this for school!