Which Nano are looking at, post a link to it.
Ensure you have FULLY inserted the USB cables.
Check you have a COMMON GROUND where required.
Where possible use USB 2.0 ports or a USB 2.0 POWERED HUB to rule out USB 3.0 issues.
Try other computers where possible.
Try other USB leads where possible.
You may not have the correct driver installed. CH340/341 or CP2102 or FT232 VCP Drivers - FTDI
There may be a problem with the board check or remove your wiring first.
Remove any items connected to pins 0 and 1.