avrdude stk500_getsync(): not in sync resp=0x30 error for Arduino Uno?/ Computer doesn't read Arduino Uno R3 anymore?

My Arduino used to work well till yesterday until I had made a mistake. I accidentally connected a 12v supply into the "5v" female header pin on the board instead of "Vin" and at the same time I had it connected to my computer usb. While uploading a sketch, I realized what mistake I had done in the wiring and pulled out the usb and disconnected the 12v supply. Since then my computer does not even read my Arduino anymore. However power LED and built in orange LED on pin 13 still turns on when I connect Arduino to my computer now.
Can any body help me?? I know I was too careless, but please help me with some solution. 

Comments

[WWC]

Replace the ATmega with a known good one that already has the bootloader installed.

[edifiedsprit]

Thanks for the reply, but according to this instructable: http://www.instructables.com/id/A-solution-to-avrdude-stk500getsync-not-in-syn/
my boot loader should be good and so should be the ATmega, however on a couple of places I had read that the USB chip 16u2 might be gone...please shed some light

[WWC]

Well whats easiest to replace first the ATmega or the " USB chip 16u2 "?

Remove the ATmega chip, then plug your USB in and look in the device manager under Ports (COM & LPT) and see it it is detected.

If it sees the Arduino then your 16u2 chip is working.

[edifiedsprit]

Ok will try that. cos there is no way to replace 16u2 by hand!