Extruder not heating

[suchigno]

I assembled my Pruse I3 Pro B, after the calibration I make my first print (plate.stl with plastic filament)
Now the extruder no longer heating.
I checked the red LED which indicates that the heating is on and when the extruder is on, it turns on. I also checked the output in volts of the door, when inactive 0.3 volts; when active 12.1 volts.
What can I do?
Thanks

[Gingko]

Hi,
Have you checked the heating wires and wires for thermistor of the extruder?
maybe they are loose.