After I tried to follow an example with stepper motor and 12V external power supplier, USB Serial port doesn't come up.
It seems the arudino couldn't recognise the board.
but when I plug in Usb cable to the board, little LEDs on the board are turned on. but I can find serial port for USB.
I think somehow it has gone wrong when i used stepper motor.
is there any way I can completely reset all the programs inside of board?