As Rixington said, it could be the fan switch. You can make the fan run with the ignition key in the on position by disconnecting the plug from the radiator thermostat and making a short between the two contacts, use a heavy gauge insulated cable or a pair of needle nose pliers or similar. If the fan runs then that can be dismissed, then reconnect the thermostsat and let the bike idle until the fan starts, it can help to put something like a white line on the outer fan so that you can see it turning.