Car not showing any sign of starting?

It's a newer car, only four years old. The other day, I was trying to start it, but the key wouldn't even turn. So we figured it was a problem with the battery. We took the battery in to be charged overnight, and the next day it worked just fine, except the stereo, which wouldn't turn on. Today, the lock-unlock button on my key wouldn't unlock the car. Manually going in, I tried to start it. The key would turn, but it wouldn't make any noise at all. I imagine it has something to do with the battery, but I can't be certain, because I'm not the best with car knowledge. Any thoughts as to what may have happened?