I have had good luck using a hot air soldering iron/gun. Do try leaving it physically unplugged for a few hours to overnight, that always worked when the same thing happened to me. The only other way is to try to clean it out with a piece of wire that is hot enough to melt and clean out the old filament. I think the extruder orifice is .4mm.

I did remove the entire print head when using the hot air solder station to unclog the unit, sadly I do recommend getting a backup print head assembly as a "plan b". they do last a pretty long time.