While wiring up my new Arduino Uno R3, I accidentally shorted the 5V pin and the GND pin (for less than a sec). It was powered by a 12V 500mAh power supply through the Arduino DC power jack. 
. The ON, Rx, X1 and L LED's are glowing continuously, and also, pressing the reset button doesn't have any effect on the LED's and arduino doesn't restarts. How can I get my arduino working again?
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Because short only lasted for a sec, maybe just maybe one of the component in the supply part of Arduino burned out. or the 500mA damaged the the main IC(ATMEGA328). so before anything change the regulator (AS1117) that is next to the power jack, and then turn on the Arduino. if it didn't work try changing the main IC. – ElectronSurf Sep 7 '19 at 4:15
The regualator of that 5V Blew up or the diode you need to read the schemtics of the Arduino Uno and check the circuits.
https://www.arduino.cc/en/uploads/Main/arduino-uno-schematic.pdf
Chances are the cheap Chinese clone board you have has an inferior 5V regulator with no thermal protection.
These are notorious for breaking with a short circuit (over-current = over-temperature = meltdown). And when they break they "fail short", which means that they end up working like a simple piece of wire - the voltage you put in is the voltage you get out.
That means that 12V has now been sent into the rest of the Arduino, and has destroyed everything - the main MCU, the USB interface chip, etc.
In short: your board is dead.
You can tell it's a fake Uno because the USB chip is not an Atmega32U4.
