Drivers are not installed or not installed correctly. Check device manager.
The most common cause of this is using a clone board with a CH340G serial chip (check part number of chip near the USB port), instead of an official one with an FT232R or ATmega16u2 as the serial interface. These boards work fine, but you need to install the CH340G drivers - search google for drivers, they're easy to find. The file is something like CH341SER.exe, dialog is super engrishy, but it works like a charm (they also happen to be harder to trash by abusing the board than the 16u2 on official Unos)
Thanks for responding but im using an official arduino uno not a clone board i asked my lecturer. Could there be any other problem with my drivers? im using arduino 1.6.5 and windows 10