Problem in upgrading ARDUINO Atmega16u2 firmware update on an Mega2560

I have arduino Mega2560 r3 board in that due to some problem when i am connecting it to system usb is not detecting ..but with another one board driver successfully install and appear in device manager.

So i tried to put that bad board to DFU mode for usb ARDUINO Atmega18u2 firmware upgrade but problem is no usb detecting so not able to upgrade it .....
please guide what should be problem in that??....

thanks in advance..

I followed the below step to put 16u2 in DFU mode as description given by arduino 16u2 DFUguide.
1 assert and hold the 8U2's RESET line
2 assert and hold the 8U2's HWB line
3 release the 8U2's RESET line
4 release the 8U2's HWB line
5. confirm that the board enumerates as either "Arduino Uno DFU" or "Arduino Mega 2560 DFU"

after completed 4th step 'L' led blink ones.. may be it indicating that it enter into DFU mode but after this USB should detect and port should come as above step 5 ...but that is not coming so please guide me what should be i am missing...??


if any one did this please suggest ..
is that am posting in wrong group???

is that problem due to usbserial ic 16u2?? it might be become bad bcz i used arduino board with SIM900 GSM shield and at one time i ddin't gave seperate 2A supply to GSM shield as suggested in forum??