12v 2000mAh battery with Arduino

Hello Arduino forum,
I am working on my first Arduino robot and need to power it via a 12v 2000mAh battery. When I plugged the - wire into the Vin port and the + wire into the GND port, the + wire fried and I opened the circuit as soon as possible. Any idea on what exactly happened and how I can prevent it?
Thank you, and I apologize if I used the forum incorrectly as it is my first time.

You connected the battery to the Arduino the wrong way round. To avoid it happening again, reverse the + and -. Also connect the battery +ve via the barrel jack instead of the Vin pin, because the barrel jack is connected via a diode to avoid damage if you use the incorrect polarity.