Arduino Nano not responding with yellow led blinking fast
I successfully restored both boards using the reset button:


Power off the board (I disconnected the USB cable from computer)
Press and hold the reset button
Connect USB cable (keep holding the reset button)
Click "Upload Sketch"
Wait a second or two until Arduino software says "Uploading..." in the status bar
Release the reset button


Use these steps to upload any small sketch, Blink example is a good choice.