I assume that you have your firmware set to use D10 for the extruder heater and you were measuring the voltage across the + and - terminals. How about when you measure the + terminal versus ground? Do you measure +12 V? Have you measured the voltage with the heater disconnected to make sure you're not shorting it? How about the resistance across the heater lead wires?
Did you measure the voltage on pin 1 of the mosfet vs ground?
Also what temperature is being read by the thermistor?
If you suspect a bad mosfet, you can change the pin assignment for the heater in the firmware to one of the other two: D8 or D9 and see if you see voltage across the +- terminals without connecting your heater.