Okay, so I was following this tutorial to control an RGB led strip with Arduino Uno R3: http://learn.adafruit.com/rgb-led-strips/usage
And the LED strip wasn't lighting, so I tried plugging the other end of the LED strip into ground. Then I realized I was using the wrong pin for ground, so I switched that and then the lights were working, but the Arduino began to overheat, and the black box next to the 12v power supply and IOREF pin became unsoldered before I had a chance to unplug the power supply. USB power works at the moment, but not 12V.
What exactly happened to cause this? Should I have not plugged in the other side of the strip into ground? What is the black box that fell off, and is there any way to solder it back on?
Edit: The part is shown here: https://www.dropbox.com/s/xu7fbycufhlq9fl/arduinopart.png?m=