one of friends accidentally supplied 36v to Vin of my uno(original Arduino UNO R3) and since then uno has stoppped to interface with my pc.
The "ON" and "L" LEDs are working but the rx/tx ones are not.
Somebody please help
If it doesn't come on as an USB device when plugged in to the PC the serial chip is dead (a ATmega on the real deal and something like a CH341 on a clone). But is that is dead the regulator passed the voltage and thus is dead. And probably the ATmega328 as well...
The ON led only tells you there is 5V (so that works).
The L led is just connected to a pin. When you say it works, what do you mean? Was the Arduino loaded with Blink an that still runs? If so, the ATmega328 is fine but the rest is broken... Easiest fix? Buy a new one on eBay 
So should replace ATmega 16U2? will that do?
And I checked the output of voltage regulator and it was giving output of 4.81 volts.
the arduino was not loaded with any blink.
And i would appreciate to know how did the serial is damaged.
And thanks for replying!!
Why do you only answer the part about blink and skip the main question? You say "the L led works" but what do you mean by that? That it's on?
And the 4,81V, is that when connected to USB? I think it is and that's no surprise. When you use the USB to power it the regulator isn't used.
But only replacing the ATmega16U2 will not do the trick.
a) Because you need to program the 16U2, it's a micro so a new bought one is empty.
b) Because I expect the voltage regulator and the ATmega328 to be broken as well...
To start the debugging, remove the ATmega32 and power the Arduino via the DC socket with something like 9V and measure the 5V rail. It's probably 9V as well (aka, regulator is broken).
To test the ATmega328 you can use another Uno or a usb-serial converter and a crystal (bare bone Arduino).
yeah that "L" LED is on.
And can I claim warranty repair from the dealer?(It's only 6 weeks old)
Thanks for replying.
nevermind, can't claim repair.
And yes the ON and L LEDs were on with 328p removed.
please help...
Find a hot air rework station and start replacing parts.
5V regulator if doesn't output 5V anymore.
3.3V regulator if doesn't output 3.3V anymore.
Atmega16U2 chip if the loopback test doesn't work after the regulators are replaced. It will need to be programmed.
Test the operation of the LM386 comparator, replace if needed.
Replace the 328P chip if it doesn't work. It will need to be bootloaded.
Part numbers can be found on the schematic, which you can find on the Products page.
Unless you have a clone, then you're on your own.
Buy a new one. You can replace the ICs, but unless you know which one is burnt, you will end up replacing all of them.
Not worth the time to fix assuming any sane value for your time. Buy a replacement.
If I were you I would tell the "friend" to buy you another UNO!!!