Electrical – Arduino is not getting power from adapter or Vin

Hi I am very much new to electronics, specially micro controllers and stuff. I am a Java/Android programmer by profession. I am currently creating a autonomous robot but I am stuck due to a problem.

The problem is that I have been using my Arduino UNO with a motor shield L293D this http://i.ebayimg.com/00/s/MTYwMFgxNjAw/z/70EAAOSwvzRXw-s7/$_12.JPG and a Bluetooth module HC05 Everything was good when I connected my shield to arduino, I checked my code using four extra small motors (2.5-3.7V). My arduino was functioning well with my code I was able to operate all the motors with it on my signal.

After all check when I added my all 4 motors to the shield in M1, M2, M3,M4 with my 12V battery of 1 AMP. There was sudden buzz and one IC L293D on motor shield got fried. Don't know this was because of some wire mess because wires that time not very tight and very neat. After that the gold thing on my arduino middle between USB and DC connector is getting very hot without even any usage. (I have replaced my ICs with new one). Now when I connected everything back all seems to be ok and my robot working fine but arduino last time drawing the power from the shield itself (12V battery pack). and now arduino is not taking power hence I am giving power to it from USB but I can't keep usb connected. So tried it with DC connector jack. It is not working too. Vin socket not working for power too. Ony usb is working. I also tried to connect my 9V battery to the 5V pin near Vin and it worked but after that HC05 stopped working but it is working fine with USB.

My Questions –

1. Why that gold thing is so hot (It wasn't happening earlier as per my memory).
2. Is my arduino got some problem if yes than can I solve it?
3. If arduino is really got fried what precautions shall I take to protect my next arduino.

Please help me with it. Thanks a lot.