Arduino MEGA 2560 R3 usb unrecognized device

Hi All,
I buy Arduino MEGA 2560 R3 2 items .And I with my brother burn program to board via USB it is OK .
I can connect USB it 's GOOD .And after i test program by many burn it is ok and last time when i burn to board my boar is led 13 ON always and CPU hot and when i connect with my computer have usb unrecognized device .And my brother board last program can use but can not connect usb with computer.

I try many solution ex. un-install driver usb and in stall again or try connect with another computer It can not connect with USB .

Please let me if anybody have solution for this issue.

Thanks,
Malscorpio