It sounds like the stepper driver may have failed if it is not allowing you to properly control the axis. I would connect a different axis motor to the z output and, taking care, manually drive the axis to see if the z axis is ok moving on that axis controller. then try another axis on the z stepper driver output to see if that fails. this will point it out.
Marchant dice no longer trade as MD, but they now run as Bionica Systems, from the same business unit.
https://www.bionicasystems.com/I have an MD unit, but a smaller A3 machine, bought years ago. i have had to replace my stepper drivers as they failed.