Z - Axis won't move down

If the Z-Axis doesn't move down this might indicate that the problem is the auto-level sensor - either it's faulty or the plug is loose.  

You can try following their suggestion:  

Turn the printer off, unplug the auto-level sensor, turn the printer on (if the Z axis is moving now without problems than it is the auto-level sensor).  

Turn the printer off and plug the auto-level sensor back in.                                                                                                                        

If the plug was just loose it should be working again. If it's still not working, the auto-level sensor is bad and it needs to be replaced.

https://gigdigit.com/auto-leveling-inductive-sensor-for-mp10-and-mp10-mini/ 

Warranty: Replacing parts on your 3D printer may result in its warranty being voided. If you are having difficulty and are unsure if your printer is still under warranty or wish to prevent voiding your warranty, please contact our technical support team for assistance.

Safety: Always exercise extreme caution when working on any electrical device, including your 3D printer. Ensure that the printer is turned off and the power plug is removed from the power outlet. Also ensure that the printer has fully cooled down prior to working on it.