Jammed extruder. Wanhao duplicator i3 Plus
I follow the techniques spelled out in this video on Tom's 3d:
Basics: Cleaning out a clogged nozzle!

Essentially you:


Heat up the hot end past the point you normally print at.
Manually (gently) push filament out of the hot end.
Turn off heat &amp; continue applying pressure to the filament, until it quits extruding (it has cooled off).
Set the heat on the hot end to normal printing temprature for the filament &amp; immediately start pulling firmly on the filament, but not too hard.
As soon as it melts enough, it will just "slip out" &amp; bring any stuck crud with it.


I went ahead and bought some white nylon filament so I can heat it up hotter (good for cleaning out ABS) and since it's white, I can see the nasty crap it pulls out.