Well, when you are entering microcontroller world, you are supposed to know some things.
One of them is that a voltage, polarity and pins the power applied to are for you to be observed, and if you mess with that, you most probably burn the thing. Sometimes with spectacular effects and even personal injury. Like high capacity LiPO battery can be quite nasty.
Even an Arduino, being quite user-friendly would let the magic smoke out if powered improperly.
I am sorry for your loss, but not much can be done here.