Did I accidentally short something? My Arduino UNO board does not communicate with the computer anymore
Unfortunately, the arduino's input pins are not tolerant of 12 volts, You have likely caused permanent damage to the chip =(. There is a regulator that makes it capable of being powered by 12 volts, but the input pins are different.

If you look at table 30-1 on the datasheet for the Uno's controller here you can see the input characteristics. The arduino is powered at 5v, so the max digital input voltage is 5.5v