This can happen if you leave the filament inside the extruder for a long period of time or shut down the printer before the nozzle is cooled down.
Preheat the nozzle to a high temperature so the filament melts down. Go to settings and change the temperature to about 240-250 degrees, preheat the nozzle.
Go back to menu and unload the filament. Sometimes you need force the filament out from the nozzle.
Cut the edge on the filament and load again. You might need to push the filament thought the nozzle.