Yesterday I was working on Arduino based bluetooth car. I was using adafruit motor shield v1. Arduino and shield were on same power supply(11v li-ion cell battery). Every thing was fine. I decided to add some leds to my car. I connected two leds with 5v on shield pins(leds were in series and were multi-colored so 5v was okay). It worked for sometime but later it stopped working. Green led on arduino was on while other were off. I disconnected shield and supplied arduino from dc jack, same result. Then I connected arduino with usb cable then everything was fine. Now it works because I supply arduino from usb and shield is working too either from usb supply or external supply. What could possibly go wrong?
It would appear that the output of the power regulator pin has been vaporized.
From the Arduino R3 schematic we see this part as U1 which is an NCP1117ST50T3G power regulator. Here is that data sheet for that part. From that data sheet we see the physical image of the part.
It appears that pin 2 has been vaporized. If we look at the schematic of the Arduino R3 we see where pin 2 supplies the regulated power to the rest of the Arduino board and likely any connected electronics. Note the red "X" indicating the vaporized pin 2.
The schematic and the data sheet both indicate the regulator's TAB is also an output. If true it is difficult to explain why pin 2 vaporized. Perhaps this particular version of the regulator does not use the TAB as an output. Or perhaps the Arduino R3 PCB and schematic do not agree.
There has been a comment that U1 (the NCP1117ST50T3G power regulator) may sometimes be manufactured with out the center pin. Still, if you can power the Arduino R3 using the USB port and unable to power the Arduino R3 using power through the barrel jack, the likely suspects are U1 and D1. However, to be sure, you would have to check the voltage before D1 and after U1.