I accidentally (stupidly) powered the uno (with adifruit motor shield stacked on it) with a 12v power supply, and about 30 seconds later it got hot and a small puff of smoke came out near the middle. I unplugged it quickly. Powered it back up with the USB cable later and all power lights come on, but it's no longer controlling my motor. What's the most probable problem, the uno, or motorshield? Sorry if this is posted in the wrong spot.
Most motor shields have a jumper or solder pads that needs to be set BEFORE use.
12 volts has also been known to break the voltage regulator on newer batches of Arduinos.
Keeping direct voltage to below 10 V for the Arduino itself seems to help most people.
Using a seperate power supply for motor shields is always a best bet but always read the documentation first.