user59318: 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?

user59169: 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

user4143: 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.

59318: <p>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. <img src="https://i.stack.imgur.com/CvYwI.jpg" alt="Now the arduino looks like this ">. 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?</p>
 59169: <p>The regualator of that 5V Blew up or the diode you need to read the schemtics of the Arduino Uno and check the circuits.</p>

<p><a href="https://www.arduino.cc/en/uploads/Main/arduino-uno-schematic.pdf" rel="nofollow noreferrer">https://www.arduino.cc/en/uploads/Main/arduino-uno-schematic.pdf</a></p>
 4143: <p>Chances are the cheap Chinese clone board you have has an inferior 5V regulator with no thermal protection.</p>

<p>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.</p>

<p>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.</p>

<p>In short: your board is dead.</p>
