First of all, my Arduino Uno R3 isn't genuine.
At some moment when I was playing with it, the Arduino stopped working. It was connected to a external 12V power supply on the 2.1mm plug. A single digital output was connected to a base of a BJT, which was being used as a switch. I checked all the voltage differences and amount of current around the BJT with my multimeter, and I made sure nothing ever exceeds the maximum rating.
One thing, I had a jumper wire connected to the 5V, and it might have accidentally got shorted with GND while poking around.
Currently, the Arduino won't start up when given external power supply soley, be it a 2.1mm plug or VIN pin. The ON light is not on. I tried to measure the current it consumes, but the value from the multimeter changes when I switch the measure range -- I suppose this is because the current is so small that the so-small resistance of the multimeter affects the current. The 5V pin, when connected to GND through a 1K ohm resistor, says about 50 mV.
However, when the USB is connected, the board works alright, regardless of whether external power supply is connected. The 5V pin says quite 5V, but I see from the schematic of Uno Rev. 3 that it is in fact just pulled from the USB power line, so it does not mean that the internal transformer circuitry is alright. When the USB gets disconnected while the USB and the external power supply is both connected, the board immediately turns off.
Am I good to regard this board as dead in some way? What do you think is broken?
And could there be any way to resusticate it?