Arduino Nano- Programer not responding after upload WDT
If the WDT program is running when you try to upload a new one, it may be blocking the bootloader. Try holding the reset button while you click the the IDE's upload button, releasing the reset once you see avrdude launch. It may take a few tries to get the button-dance just right.

Update:


  what is avrdude ?


It is the program on your PC that uploads the compiled image to the Arduino. You'll see it running in the lower pane of the Arduino IDE.