Did you check for power at the 16u2?
Yep, there is 5V.
How did this happen?
I have a board did be me (based on mega protoshield), which is connected to the arduino. To this board I have attached a lot of connections to the external devices like servos, potentiometers, etc. and of course for power.
So, one of devices has wrong connection, because works strange (I have that earlier).
I unpluged, fixed ("bum" here), plug, and I don't double check power lines, and yes it was wrong. After that I saw a lot of smoke from arduino;-/
Maybe it burned some traces that supply power to the 16u2. 5V or ground.
Traces looks good, but I saw that chip stays hot when power is plugged.
Arrrggghh... It will be hard (or maybe impossible for me) to fix it in home...
EDIT:
CLOSED, now it's really burned...;-/