ARDUINO UNO R3 not recognized

My arduino uno was working fine...but all of a sudden it stopped working..now it z not being recognized by my pc or laptop... it z not being shown in the device manager...but as soon as i connect it to the pc...the power on led and led13 are glowing..i also tried resetting ATMEGA328 but to no avail...i am completely lost...HELP ME!!!!

Could be a driver issue... you might try the alternative IDE installer developed by ereid. Awesome sauce.

If you can get a COM port for the board, but you only see "ATmega16u2 DFU" in Device Manager, you may need to reprogram the 16u2 chip. There are some outdated instructions here, in the Communications section. That section has a reference to a tutorial on programming the 8u2 chip, so I'm not sure how helpful it will be to you. If the 16u2 chip is DFU mode, you may be able to program it with FLIP, using this hex file:

Arduino-usbserial-atmega16u2-Uno-Rev3.hex

It should be in your Arduino install directory, probably something like

C:\Program Files\arduino-1.0.3-windows\hardware\arduino\firmwares\atmegaxxu2\arduino-usbserial

I had mixed results using FLIP. However, I was able to reprogram it with an AVRISPmkii. You will need an ISP or second functioning Arduino programmed to act like an ISP. See the first avrdude command here, but use the hex file above. Once it's programmed, it will no longer be in DFU mode, and you should probably be able to see your UNO board listed in the COM ports.

After that, if you still can't upload sketches, it is possible that the 328 has been wiped (i.e., it does not have a bootloader). I cannot determine from the official pages what you should upload to the ATmega328p. Someone else is sure to chime in. :)

Cheers, /dev

I had a similar problem a while ago with my Arduino UNO R3. I confirm that it is probably a problem with the ATmega328p chip. You probably burnt it when you exposed the GND to 5V pin for a long time (that's what happened to mine).

Here is alink to topic I've made a while ago: http://arduino.cc/forum/index.php/topic,137888.msg1035962.html#msg1035962

Thank you /dev and mixania....but my problem isnt solved.. i have two arduino uno's...and only one z not being recognized..so it means that there z no problem with the drivers right..?? And i dont think that it z because of exposing my ground and vcc for long time because..in that case..my MCU must have been burned up...but even after changing the MCU (which also has atmega 328 opti boot loader)...no improvement...and in the device manager i cant see anything that indicates my arduino is connected..neither atmega 16u2 nor anything else..what to do???

What worked for me was that I first installed the driver that came with it, restarted my computer, plugged in my arduino and turns out Windows Update had a driver for it.

Windows x64

Have you tried other USB cables?

How about burning the example AsciiTable in the good board, then move the chip to the bad board. Do the TX/RX LEDs flicker?

Cheers,
/dev

thank you striker_007 and /dev..but still the problem persists..i have tried what u have told even before and now again..but to no avail..