I had a similar situation happen while testing a motor. Stupid me, yes. Same chip, saw some smoke come out of it. Yikes! I've learned my lesson and will be more careful.
Question: When I plug in my mega2560 (both with a 12v power supply and with USB), the "On" and "L" or "13" LEDs turn on. They don't blink, though. When I push the reset button, the "L" LED blinks, and then just holds solid. Windows 7 32-bit won't recognize it. This is the same computer I've used for a long time. I'll order some of the AMS1117 chips and replace the one that burned. Should the Arduino be recognized by Windows even without AMS1117 chip functioning?