Unplug the USB cable for your external drive. Go to device manager and open the category with your drive, generally, disk drives. If the device is still listed, right click on it and select uninstall. Reboot and once windows is loaded plug the device back in and check to see if it is now recognized.
If no device was listed in device manager when you unplugged the USB cable, go up to "View" at the top of device manager and select "show hidden devices". Go to disk drives and see if the drive shows up as a grayed out device. If so, perform the right click uninstall as above. If not, just reboot and then plug it back in once back in windows. If none of this works, run Seatools for windows on the drive to see if there is a problem with the device. Run the short DST (Drive self test) and if it passes, run the long generic.
Seatools for windows:
http://www.seagate.com/support/downloads/item/seatools-win-master/