Too little info.
However, here's one free hint: if you're using Linux, then you can't (AFAIK) upload to the Uno via the USB ports without first allowing access to the ports: sudo chmod 777 /dev/ttyACM* will get 'em all in one blow. Change the USB* to USB* if you're using the Nano.
At least, that's the way it works on my computer. You may have to repeat the above whenever you unplug and re-plug the USB cable(s).