My external power is good for sure. Power indicator is on.
If I hook the usb cable back, the led starts working again.
I think when we upload code using arduino ide through the usb cable, a HEX file is generated and stored somewhere in the computer. The arduino board then pull down the hex file from computer to control led. Am I correct?
So, we need to burn the hex file to the arduino chip for the board to run itself, but how?