Apply boatloader mode recovery option:
- Double tap the reset button on your board quickly. You should now see the LED on the board pulsing (fade in and out slowly), which means the bootloader is running. The double press/tap makes the bootloader to run indefinitely (until the board is reset by taping again the rest button once, powered off, or an upload is done).
- Select the COM port for the board from the Tools > Port menu. The port number is usually different when the bootloader is running.
- Start compile and upload a Blink LED sketch from the Arduino IDE examples.