Arduino Nano - USB not present after reboot
Pin 19 (RESET) of the FT232RL (USB driver) seems to be unconnected according to Nano's circuit diagram. 

Double check if this is indeed the case. If so, try connecting a 100nF capacitor from that pin to ground and a 10k resistor to +5V. This may reset the device long enough for the USB host (PC) to initialize.







simulate this circuit &ndash; Schematic created using CircuitLab