German RepRep Neo can't heat the extruder
first, check the breakers 

pic source

secondary stuff

please see a list of item to check, that could point you to the source:


heater cable cut/melted/loose => the best way will be to check the resistance (it shall be around 3.3 Ohms) from the heater cable terminals on the mainboard, if that fails it could be a wire on heater itself.
with the heating switch on - measure the voltage on terminals - shall be 12V, if not  - that  could suggest a mainboard failure or mosfet failure (in case of mosfet you could measure if there is an input signal given on the middle pin)
there also could be a breaker that was engaged due to short circuit on the wires (the main board layout is not known to me and it is not clear form manual I found online)