Question about the MK8 Extruder
It’s either because you haven’t connected it properly or you have not  mounted it the correct way. This could lead to the damaging of the part and failure to work again. 

Check your connections with the stepper motors and hotend power wires. Make sure the connectors are in fine condition and cords are not frayed. Ensure all power is completely off and mains cable is unplugged.