If you have an older version of Windows or simply unable to connect to the Arduino Uno/Mega/Nano through the IDE then it may be that you need to install the CH340 drivers.  

If you are having problems uploading a sketch to the Arduino Nano then be sure to first update the "Arduino AVR Boards" under the menu Tools-> Board -> Boards Manager

If you still having problems then try using the "Old Bootloader" options under Tools-> Processor"

Download the drivers at https://leobot.net/Arduino_CH340.zip