Who has shorted the 5Vdc supply to GND on the Nano? And now it does not even show the power LED, or it is on very dim? You simply destroyed the MBR0520 diode, labeled D1 in the " +5V Auto Selector " circuit on the board. How to fix? Either order another replacement MBR0520LT1G, 0.5A, 20V, Schottky Diode, or promise yourself to never, ever short Vcc to GND again and solder a jumper in! Diode labeled B2 Getting less expensive Arduinos from Ebay? Well, a lot of times the USB connector is not soldered properly and you get intermittent power or data to the ATMEGA chip. How to fix? Touch all the pins up with a solder iron and flux.