problem with arduino nano

hi every one
i have arduino nano
it was work more than one time
but suddenly when i connect it with my computer
it give me (usb unknown device message) and can't give me the com port
and the L led in my arduino keep flashing not stop

please help

same problem bro don't know how to fix it

The Nano uses a FTDI chip for USB to Serial comms.

Have you installed the FTDI drivers:
http://www.ftdichip.com/Drivers/VCP.htm

If so, it could be a bad USB cable or flaky USB cable connection. Try a different USB cable.

thanks for reply
but the cable is fine
the driver file is fine
and i bought anew nano Bord it's work fine
with the same cable the same pc

relay it is bad luck
the other one i bought get the same problem
it work fine on Bord but can't read on pc